Publish id().
Implement map() as a reducer.
Group reducers in the file navigator.
Use _ for unused parameters.
Implement filter() as a reducer.
Document filter().
Convenience predicates.
Test all-inclusive filters.
Test all-exclusive filters.
Test semi-inclusive filters.
Implement flattenMap() as a reducer.
Implement flattenMap() with maps to sequences.
Test flattenMap with id (i.e. concat).
Implement concat() as API wrapping flattenMap().
Variable names should reflect their intent.
Test flattenMap as filter.
ReducibleOf can be initialized with one element.
ReducibleOf can be initialized empty.
Replace FilterReducibleView with flattenMap().
Test flattenMap as map.