diff --git a/DESCRIPTION b/DESCRIPTION
index 067dcb630cb34a0be949cbf33d07ecd6a640525b..8610a2b08012c934edfcb5b64b782a98315e9f1d 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -1,8 +1,8 @@
 Package: stressaddition
 Type: Package
 Title: Modeling Tri-Phasic Concentration-Response Relationships
-Version: 2.2.0
-Date: 2020-02-28
+Version: 2.2.1
+Date: 2020-03-03
 Authors@R: c(person("Sebastian", 
                     "Henz", 
                     role = c("aut", "cre"), 
diff --git a/NEWS.md b/NEWS.md
index 1379a56a09ad3cca342561132b776270b57c6029..b954e4d6be1cda2075ff18af1db1fa5baebe4fde 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,3 +1,7 @@
+# stressaddition 2.2.1
+
+* Improve documentation of `predict_mixture()` and include example of symmetry.
+
 # stressaddition 2.2.0
 
 * `ec()` raises an error if the curve does not cross the desired response level.
diff --git a/R/ec.R b/R/ec.R
index 5fb444b30e798ce454c945ff604d288585a22fb1..69c9da6740adf5cb0de2746748de57ba96a3dbcc 100644
--- a/R/ec.R
+++ b/R/ec.R
@@ -1,3 +1,22 @@
+# Copyright (C) 2020  Helmholtz-Zentrum fuer Umweltforschung GmbH - UFZ
+# See file inst/COPYRIGHTS for details.
+#
+# This file is part of the R package stressaddition.
+#
+# stressaddition is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+
 #' Effect Concentrations
 #'
 #' Estimate the concentration to reach a certain effect or stress level relative
diff --git a/R/ecxsys.R b/R/ecxsys.R
index ccab533eafee1993663dd74c0e14e9504273c47c..d6ba1a7a4618efa91a3c32ed96bf5587487bda30 100644
--- a/R/ecxsys.R
+++ b/R/ecxsys.R
@@ -1,3 +1,22 @@
+# Copyright (C) 2020  Helmholtz-Zentrum fuer Umweltforschung GmbH - UFZ
+# See file inst/COPYRIGHTS for details.
+#
+# This file is part of the R package stressaddition.
+#
+# stressaddition is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+
 # Note about the setting and resetting of options:
 # The drc package changes some of the options which some users may have
 # modified. Of particular interest is options("warn") because it is important
diff --git a/R/helpers.R b/R/helpers.R
index a34ae96fcbe3874d3cf61b3f3019e6d5d37f4129..acdfaff8f3338c13dd67c7e018cc29e5c872d740 100644
--- a/R/helpers.R
+++ b/R/helpers.R
@@ -1,4 +1,23 @@
-# A collection of internal helper functions which get used in more than one place.
+# Copyright (C) 2020  Helmholtz-Zentrum fuer Umweltforschung GmbH - UFZ
+# See file inst/COPYRIGHTS for details.
+#
+# This file is part of the R package stressaddition.
+#
+# stressaddition is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+
+# A collection of some internal helper functions.
 
 
 clamp <- function(x, lower = 0, upper = 1) {
diff --git a/R/plot_ecxsys.R b/R/plot_ecxsys.R
index 18a31e67edf2e11215e40f180023411f7498de84..52a812ca66b4c61da63c89f5af78837130691d05 100644
--- a/R/plot_ecxsys.R
+++ b/R/plot_ecxsys.R
@@ -1,4 +1,23 @@
-# This is the shared documentation for the plotting functions
+# Copyright (C) 2020  Helmholtz-Zentrum fuer Umweltforschung GmbH - UFZ
+# See file inst/COPYRIGHTS for details.
+#
+# This file is part of the R package stressaddition.
+#
+# stressaddition is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+
+# This is the shared documentation for the plotting functions.
 
 
 #' Plot the results of the ECx-SyS model
diff --git a/R/plot_effect.R b/R/plot_effect.R
index b9075a1f12fb7a1fe86f8cc8307ab3937637fe9a..5a0da15f0b2de26ec4e4f6bd12fd31b8b5f8542b 100644
--- a/R/plot_effect.R
+++ b/R/plot_effect.R
@@ -1,3 +1,22 @@
+# Copyright (C) 2020  Helmholtz-Zentrum fuer Umweltforschung GmbH - UFZ
+# See file inst/COPYRIGHTS for details.
+#
+# This file is part of the R package stressaddition.
+#
+# stressaddition is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+
 #' @rdname plot_ecxsys
 #' @export
 plot_effect <- function(model,
diff --git a/R/plot_stress.R b/R/plot_stress.R
index 62979cdcc97f32c3c672ba96c1013eb67f17b159..ee80cd49846519fae905291dd778af08b160c2c8 100644
--- a/R/plot_stress.R
+++ b/R/plot_stress.R
@@ -1,3 +1,22 @@
+# Copyright (C) 2020  Helmholtz-Zentrum fuer Umweltforschung GmbH - UFZ
+# See file inst/COPYRIGHTS for details.
+#
+# This file is part of the R package stressaddition.
+#
+# stressaddition is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+
 #' @rdname plot_ecxsys
 #' @export
 plot_stress <- function(model,
diff --git a/R/predict_ecxsys.R b/R/predict_ecxsys.R
index 7d3d82a5a48eac83aff288cc68771a56ba007272..567ac6705c655036cb1815e342dc27fe3a8f9107 100644
--- a/R/predict_ecxsys.R
+++ b/R/predict_ecxsys.R
@@ -1,3 +1,22 @@
+# Copyright (C) 2020  Helmholtz-Zentrum fuer Umweltforschung GmbH - UFZ
+# See file inst/COPYRIGHTS for details.
+#
+# This file is part of the R package stressaddition.
+#
+# stressaddition is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+
 #' Predict effects and stresses
 #'
 #' Calculate the effects and stresses of an ECx-SyS model at arbitrary
diff --git a/R/predict_mixture.R b/R/predict_mixture.R
index a9d343451e40f22d18acbd5a4ee66f706d6612a5..f70f6a60e381d8cdd0de15759dd3ac24a83fe7e3 100644
--- a/R/predict_mixture.R
+++ b/R/predict_mixture.R
@@ -1,5 +1,35 @@
+# Copyright (C) 2020  Helmholtz-Zentrum fuer Umweltforschung GmbH - UFZ
+# See file inst/COPYRIGHTS for details.
+#
+# This file is part of the R package stressaddition.
+#
+# stressaddition is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+
 #' Predict the effect of a mixture of two toxicants
 #'
+#' Given the ecxsys models of two toxicants this method predicts the effects of
+#' different mixtures of both.
+#'
+#' The prediction happens at one or multiple concentrations of toxicant 1 and
+#' one concentration of toxicant 2. This allows for easy plotting over the
+#' concentrations of toxicant 1.
+#'
+#' The predictions are symmetric, i.e. it does not matter which of the toxicant
+#' models is 1 or 2 as long as the concentration arguments are supplied
+#' in the right order. See the example below.
+#'
 #' This method is only suitable for experiments without environmental stress.
 #' Any environmental stress in \code{model_1} or \code{model_2} is ignored.
 #'
@@ -13,23 +43,39 @@
 #' @return A vector of the effects of the mixture, scaled to the effect_max
 #'   value of model_1.
 #'
-#' @examples toxicant_model_1 <- ecxsys(
+#' @examples toxicant_1  <- ecxsys(
 #'     concentration = c(0, 0.05, 0.5, 5, 30),
 #'     hormesis_concentration = 0.5,
 #'     effect_tox_observed = c(90, 81, 92, 28, 0),
 #' )
-#' toxicant_model_2 <- ecxsys(
+#' toxicant_2  <- ecxsys(
 #'     concentration = c(0, 0.1, 1, 10, 100, 1000),
 #'     hormesis_concentration = 10,
 #'     effect_tox_observed = c(26, 25, 24, 27, 5, 0),
 #'     effect_max = 30
 #' )
 #' predict_mixture(
-#'     toxicant_model_1,
-#'     toxicant_model_2,
+#'     toxicant_1 ,
+#'     toxicant_2 ,
 #'     c(0, 0.02, 0.2, 2, 20),
 #'     3
 #' )
+#'
+#' # Example of symmetric prediction:
+#' conc_1 <- c(0, 0.03, 0.3, 3)
+#' conc_2 <- 5.5
+#' prop_ca <- 0.75
+#' effect_a <- predict_mixture(toxicant_1 , toxicant_2 , conc_1, conc_2, prop_ca)
+#' effect_b <- sapply(
+#'     conc_1,
+#'     function(x) predict_mixture(toxicant_2 , toxicant_1 , conc_2, x, prop_ca)
+#' )
+#' # The effect_max values of the models are different. effect_b is scaled to
+#' # the one from toxicant 2 but to compare the results effect_b must be scaled
+#' # to the effect_max of toxicant 1:
+#' effect_b <- effect_b / toxicant_2 $args$effect_max * toxicant_1$args$effect_max
+#' identical(effect_a, effect_b)
+#'
 #' @export
 predict_mixture <- function(model_1,
                             model_2,
diff --git a/R/stress_effect_conversion.R b/R/stress_effect_conversion.R
index ef6c252dbfc73d317fa09f863ef0a7339fc7cf17..49dcc5bee474f69fa2ec6c2646f09890841c8aae 100644
--- a/R/stress_effect_conversion.R
+++ b/R/stress_effect_conversion.R
@@ -1,3 +1,22 @@
+# Copyright (C) 2020  Helmholtz-Zentrum fuer Umweltforschung GmbH - UFZ
+# See file inst/COPYRIGHTS for details.
+#
+# This file is part of the R package stressaddition.
+#
+# stressaddition is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+
 #' Convert Between Stress and Effect
 #'
 #' Functions to convert effect to general stress or vice versa using the beta
diff --git a/R/stressaddition-package.R b/R/stressaddition-package.R
index 16dd5a0e6bbfcfda8ecde00bde026d6f22817745..abd7efacbf55d6e5cb8a4e8ba5a093e390c8239f 100644
--- a/R/stressaddition-package.R
+++ b/R/stressaddition-package.R
@@ -1,6 +1,26 @@
+# Copyright (C) 2020  Helmholtz-Zentrum fuer Umweltforschung GmbH - UFZ
+# See file inst/COPYRIGHTS for details.
+#
+# This file is part of the R package stressaddition.
+#
+# stressaddition is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+
 # The help file for the package, accessible via ?stressaddition and
 # ?`stressaddition-package`. Some people recommend using "@keywords internal"
-# here to exclude this page from the help index but I'm not going to do that.
+# here to exclude this page from the help index. But I'm not going to do that
+# because I want to keep it on the index.
 
 # "_PACKAGE" automatically populates most sections in this package
 # documentation, so I don't need to manually fill in authors, title or
diff --git a/README.md b/README.md
index 7ffe0649aa53712860fabd0beeebf193674c3358..426439e62241bc939ef227cc15fc779c37fa4fc7 100644
--- a/README.md
+++ b/README.md
@@ -43,3 +43,32 @@ ec(model, "effect_tox_sys", 50)
 # The LC10 of the effect under the influence  of toxicant, system and environmental tress:
 ec(model, "effect_tox_env_sys", 10)
 ```
+
+## Copyright and License
+Copyright (c) 2020,  
+Helmholtz-Zentrum fuer Umweltforschung GmbH - UFZ.  
+All rights reserved.
+
+The code is a property of:
+
+Helmholtz-Zentrum fuer Umweltforschung GmbH - UFZ  
+Registered Office: Leipzig  
+Registration Office: Amtsgericht Leipzig  
+Trade Register: Nr. B 4703  
+Chairman of the Supervisory Board: MinDirig'in Oda Keppler  
+Scientific Director: Prof. Dr. Georg Teutsch  
+Administrative Director: Dr. Sabine König  
+
+
+stressaddition is free software: you can redistribute it and/or modify  
+it under the terms of the GNU General Public License as published by  
+the Free Software Foundation, either version 3 of the License, or  
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,  
+but WITHOUT ANY WARRANTY; without even the implied warranty of  
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the  
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License  
+along with this program.  If not, see <https://www.gnu.org/licenses/>.
diff --git a/man/predict_mixture.Rd b/man/predict_mixture.Rd
index bc43850edd096f54a6841cf3d35623ca6920fc86..8595ac80da97ffe27d7fcc38bd4e9361212800cc 100644
--- a/man/predict_mixture.Rd
+++ b/man/predict_mixture.Rd
@@ -29,25 +29,53 @@ A vector of the effects of the mixture, scaled to the effect_max
   value of model_1.
 }
 \description{
+Given the ecxsys models of two toxicants this method predicts the effects of
+different mixtures of both.
+}
+\details{
+The prediction happens at one or multiple concentrations of toxicant 1 and
+one concentration of toxicant 2. This allows for easy plotting over the
+concentrations of toxicant 1.
+
+The predictions are symmetric, i.e. it does not matter which of the toxicant
+models is 1 or 2 as long as the concentration arguments are supplied
+in the right order. See the example below.
+
 This method is only suitable for experiments without environmental stress.
 Any environmental stress in \code{model_1} or \code{model_2} is ignored.
 }
 \examples{
-toxicant_model_1 <- ecxsys(
+toxicant_1  <- ecxsys(
     concentration = c(0, 0.05, 0.5, 5, 30),
     hormesis_concentration = 0.5,
     effect_tox_observed = c(90, 81, 92, 28, 0),
 )
-toxicant_model_2 <- ecxsys(
+toxicant_2  <- ecxsys(
     concentration = c(0, 0.1, 1, 10, 100, 1000),
     hormesis_concentration = 10,
     effect_tox_observed = c(26, 25, 24, 27, 5, 0),
     effect_max = 30
 )
 predict_mixture(
-    toxicant_model_1,
-    toxicant_model_2,
+    toxicant_1 ,
+    toxicant_2 ,
     c(0, 0.02, 0.2, 2, 20),
     3
 )
+
+# Example of symmetric prediction:
+conc_1 <- c(0, 0.03, 0.3, 3)
+conc_2 <- 5.5
+prop_ca <- 0.75
+effect_a <- predict_mixture(toxicant_1 , toxicant_2 , conc_1, conc_2, prop_ca)
+effect_b <- sapply(
+    conc_1,
+    function(x) predict_mixture(toxicant_2 , toxicant_1 , conc_2, x, prop_ca)
+)
+# The effect_max values of the models are different. effect_b is scaled to
+# the one from toxicant 2 but to compare the results effect_b must be scaled
+# to the effect_max of toxicant 1:
+effect_b <- effect_b / toxicant_2 $args$effect_max * toxicant_1$args$effect_max
+identical(effect_a, effect_b)
+
 }
diff --git a/tests/testthat.R b/tests/testthat.R
index 1f87a9a9c20da7a69b927240ded0742051c95a6d..d350fe5adfc8e4013f79c439bb975924ac5ceff2 100644
--- a/tests/testthat.R
+++ b/tests/testthat.R
@@ -1,3 +1,22 @@
+# Copyright (C) 2020  Helmholtz-Zentrum fuer Umweltforschung GmbH - UFZ
+# See file inst/COPYRIGHTS for details.
+#
+# This file is part of the R package stressaddition.
+#
+# stressaddition is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+
 library(testthat)
 library(stressaddition)
 
diff --git a/tests/testthat/test-ec.R b/tests/testthat/test-ec.R
index 138fbcedafabfcdfa9bc1508bb6d7a2fc73a048e..0371c7d868c98e881cc7dc79309a44871a78c467 100644
--- a/tests/testthat/test-ec.R
+++ b/tests/testthat/test-ec.R
@@ -1,3 +1,22 @@
+# Copyright (C) 2020  Helmholtz-Zentrum fuer Umweltforschung GmbH - UFZ
+# See file inst/COPYRIGHTS for details.
+#
+# This file is part of the R package stressaddition.
+#
+# stressaddition is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+
 test_that("all input formats produce identical models", {
     model <- ecxsys(
         concentration = c(0, 0.05, 0.5, 5, 30),
diff --git a/tests/testthat/test-ecxsys.R b/tests/testthat/test-ecxsys.R
index 9911f1d825f21f80ac2989531442ef68e1906c00..01dcd29941a74c9616a370f3fd537601a2665b6f 100644
--- a/tests/testthat/test-ecxsys.R
+++ b/tests/testthat/test-ecxsys.R
@@ -1,3 +1,22 @@
+# Copyright (C) 2020  Helmholtz-Zentrum fuer Umweltforschung GmbH - UFZ
+# See file inst/COPYRIGHTS for details.
+#
+# This file is part of the R package stressaddition.
+#
+# stressaddition is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+
 mod <- ecxsys(
     concentration = c(0, 0.05, 0.5, 5, 30),
     hormesis_concentration = 0.5,
diff --git a/tests/testthat/test-options.R b/tests/testthat/test-options.R
index 6d7c85d1db31a21e9bd235a7c173fcb26fd6b8e1..2ff4a81632c9269b53a79b06c98ffb4054376f4f 100644
--- a/tests/testthat/test-options.R
+++ b/tests/testthat/test-options.R
@@ -1,3 +1,22 @@
+# Copyright (C) 2020  Helmholtz-Zentrum fuer Umweltforschung GmbH - UFZ
+# See file inst/COPYRIGHTS for details.
+#
+# This file is part of the R package stressaddition.
+#
+# stressaddition is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+
 test_that("user options are not permanently changed by ecxsys()", {
     # drc::drm() messes with the options and fails to return them to their
     # previous values. And options are temporarily modified in ecxsys(). This
diff --git a/tests/testthat/test-plot_ecxsys.R b/tests/testthat/test-plot_ecxsys.R
index 4a61b99d27c535b04eef909476a1bf6e128d6232..a0dea0a1d6556b76502c4c736e53120584fc2964 100644
--- a/tests/testthat/test-plot_ecxsys.R
+++ b/tests/testthat/test-plot_ecxsys.R
@@ -1,3 +1,22 @@
+# Copyright (C) 2020  Helmholtz-Zentrum fuer Umweltforschung GmbH - UFZ
+# See file inst/COPYRIGHTS for details.
+#
+# This file is part of the R package stressaddition.
+#
+# stressaddition is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+
 test_that("log ticks are correct", {
   x <- c(0.03, 0.3, 3, 30)
   ticks <- get_log_ticks(x)
diff --git a/tests/testthat/test-predict_mixture.R b/tests/testthat/test-predict_mixture.R
index 3471aeb429a0e7cff5f1496d08445e36253eb99f..715697f791ee352f83cbf77a79c15fbd8ff880d1 100644
--- a/tests/testthat/test-predict_mixture.R
+++ b/tests/testthat/test-predict_mixture.R
@@ -1,3 +1,22 @@
+# Copyright (C) 2020  Helmholtz-Zentrum fuer Umweltforschung GmbH - UFZ
+# See file inst/COPYRIGHTS for details.
+#
+# This file is part of the R package stressaddition.
+#
+# stressaddition is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+
 model_1 <- ecxsys(
     concentration = c(0, 0.05, 0.5, 5, 30),
     hormesis_concentration = 0.5,
diff --git a/tests/testthat/test-stress_effect_conversion.R b/tests/testthat/test-stress_effect_conversion.R
index b583ece438293fb19df092d467a5b016054c6c98..e17a2ae4e049c3e33ef3f44d647e3a1367b8a747 100644
--- a/tests/testthat/test-stress_effect_conversion.R
+++ b/tests/testthat/test-stress_effect_conversion.R
@@ -1,3 +1,22 @@
+# Copyright (C) 2020  Helmholtz-Zentrum fuer Umweltforschung GmbH - UFZ
+# See file inst/COPYRIGHTS for details.
+#
+# This file is part of the R package stressaddition.
+#
+# stressaddition is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+
 test_that("stress_to_effect handles bad input", {
     expect_equal(stress_to_effect(-3), 1)
     expect_equal(stress_to_effect(10), 0)