Problem: Functions implementing reduce and map are too long.
Solution: Use a function for each type of value. Add a few more test cases
and add to the help. (Yegappan Lakshmanan, closesvim/vim#9370)
389b72196e
Partial port as this doesn't include handling for non-materialized List.
Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>