Misc_stdlib.Listis_empty l is true if and only if l has no elements. It is equivalent to compare_length_with l 0 = 0.
val merge_fold :
cmp:('a -> 'b -> int) ->
left_only:('acc -> 'a -> 'acc) ->
right_only:('acc -> 'b -> 'acc) ->
both:('acc -> 'a -> 'b -> 'acc) ->
init:'acc ->
'a list ->
'b list ->
'accFolds over two sorted lists, calling left_only on those elements that appear only in the left list, right_only on those elements that appear only in the right list, and both on those elements that appear in both.