function calls. Instead of copying the entire dios, we now only
store the values we actually masked and their `iloc`-position.
This has the same worst-case-costs (i.e. all values are subject
to masking) but results in significantly cheaper average costs
(i.e. only a small subset of values are subject to masking)

the word 'Flagger' now is History, we have 'Flags' now (except for plotting, which is old and deaf and probably die soon anyway)