Update GenericFunctions.md

docs/GenericFunctions.md

 # Generic Functions
-Generic Functions provide a possibility to implement simple quality checks
-directly within the configuration using a simple, Python based extension 
-language.
+
+Generic Functions provide a way to leverage cross-variable conditions 
+and to implement simple quality checks directly within the configuration.
+
+## Why?
+The underlying idea is, that in most real world datasets many errors
+can be explained by the dataset itself. Think of a an active, fan-cooled
+measurement device: no matter how precise the instrument may work, problems
+are to expected when the fan stop working or the battery voltage 
+drops below a certain threshold. While these dependencies are easy to 
+[formalize](#a-real-world-example) on a per dataset basis, it is quite
+challenging to translate them into general purpose source code.
 
 ## Specification
-Generic funtions are used in the same manner as their
+Generic functions are used in the same manner as their
 [non-generic counterparts](docs/FunctionDescriptions.md). The basic 
 signature looks like that:
 ```sh
 flagGeneric(func=<expression>, flag=<flagging_constant>)
 ```
 where `<expression>` is composed of the [supported constructs](#supported-constructs)
-and `<flag_constant>` is either one of the predefined
-[flagging constants](docs/ParameterDescriptions.md#flagging-constants) or any value supported
-by the flagger in use.
+and `<flag_constant>` is one of the predefined
+[flagging constants](docs/ParameterDescriptions.md#flagging-constants) (default: `BAD`)
+
 
 ## Examples
 
 ### Simple comparisons
 
 #### Task
-Flag all values of variable `x` when variable `y` falls below a certain threashold
+Flag all values of variable `x` when variable `y` falls below a certain threshold
 
 #### Configuration file
 
-| varname | test                      |
-|---------|---------------------------|
-| `x`     | `flagGeneric(func=y < 0)` |
+| varname | test                    |
+|---------|-------------------------|
+| x       | flagGeneric(func=y < 0) |
 
 ### Calculations
 
@@ -35,9 +44,9 @@ Flag all values of variable `x` that exceed 3 standard deviations of variable `y
 
 #### Configuration file
 
-| varname | test                                  |
-|---------|---------------------------------------|
-| `x`     | `flagGeneric(func=this > std(y) * 3)` |
+| varname | test                                |
+|---------|-------------------------------------|
+| x       | flagGeneric(func=this > std(y) * 3) |
 
 ### Special functions
 
@@ -46,20 +55,53 @@ Flag variable `x` where variable `y` is flagged and variable `x` has missing val
 
 #### Configuration file
 
-| varname | test                                                  |
-|---------|-------------------------------------------------------|
-| `x`     | `flagGeneric(func=this > isflagged(y) & ismissing(z)` |
+| varname | test                                                |
+|---------|-----------------------------------------------------|
+| x       | flagGeneric(func=this > isflagged(y) & ismissing(z) |
+
+
+### A real world example
+Let's consider a dataset like the following:
+
+| date             | meas | fan | volt |
+|------------------|------|-----|------|
+| 2018-06-01 12:00 | 3.56 |   1 | 12.1 |
+| 2018-06-01 12:10 |  4.7 |   0 | 12.0 |
+| 2018-06-01 12:20 |  0.1 |   1 | 11.5 |
+| 2018-06-01 12:30 | 3.62 |   1 | 12.1 |
+| ...              |      |     |      |
+
+#### Task
+Flag variable `meas` where variable `fan` equals 0 and variable `volt`
+is lower than `12.0`.
+
+#### Configuration file
+We can directly implement the condition as follows:
+
+| varname | test                                         |
+|---------|----------------------------------------------|
+| meas    | flagGeneric(func=(fan == 0)  (volt < 12.0)) |
+
+But we could also quality check our independent variables first
+and than leverage this information later on:
+
+| varname | test                                                    |
+|---------|---------------------------------------------------------|
+| *       | missing()                                               |
+| fan     | flagGeneric(func=this == 0)                             |
+| volt    | flagGeneric(func=this < 12.0)                           |
+| meas    | flagGeneric(func=isflagged(fan) &vert; isflagged(volt)) |
 
 
 ## Variable References
-All variables of the processed dataset are available within generic functions, so 
-arbitrary cross references are possible. The variable of intereset 
+All variables of the processed dataset are available within generic functions,
+so arbitrary cross references are possible. The variable of interest 
 is furthermore available with the special reference `this`, so the second 
 [example](#calculations) could be rewritten as: 
 
-| varname | test                               |
-|---------|------------------------------------|
-| `x`     | `flagGeneric(func=x > std(y) * 3)` |
+| varname | test                             |
+|---------|----------------------------------|
+| x       | flagGeneric(func=x > std(y) * 3) |
 
 When referencing other variables, their flags will be respected during evaluation
 of the generic expression. So, in the example above only previously
@@ -70,7 +112,7 @@ unflagged values of `x` and `y` are used within the expression `x > std(y)*3`.
 
 ### Operators
 
-#### Comparsions
+#### Comparison
 
 The following comparison operators are available:
 | Operator | Description                                                                                        |
@@ -82,15 +124,15 @@ The following comparison operators are available:
 | `>=`     | `True` if the values of the left operand are greater or equal than the values of the right operand |
 | `<=`     | `True` if the values of the left operand are smaller or equal than the values of the right operand |
 
-#### Arithmetics
+#### Arithmetic
 The following arithmetic operators are supported:
 | Operator | Description    |
 |----------|----------------|
 | `+`      | addition       |
-| `-`      | substraction   |
+| `-`      | subtraction    |
 | `*`      | multiplication |
 | `/`      | division       |
-| `**`     | exponantion    |
+| `**`     | exponentiation |
 | `%`      | modulus        |
 
 #### Bitwise