Maps over Unicode characters.
map ?eq f m and
mapi ?eq f m : Similar to
in stdlib Map, but if the map
m' is returned, it is only guaranteed
eq (find u m') (f (find u m )) is true for
eq (find u m') (f u (find u m )) is true for
not specified, structural equality is used.
fold_range f m x is equivalent to
f u_(2n) u_(2n+1) v_n (... (f u_1 u_2 v_1 x)) where all characters in
u_(2k+1) are mapped to
u_3 < ... in code point order.
For each range
u_(2k+1) is separated by a character
which is not mapped to