Sets of Unicode characters, implemented as sets of intervals. The signature is mostly same to Set.S in stdlib
fold_range f s x
is equivalent to
f u_i u_(i+1) (... (f u_3 u_4 (f u_1 u_2 x)))
if s
is consisted of
the intervals u1
-u2
, u3
-u4
, ..., u_i
-u_(i + 1)
in increasing order. The intervals given to proc
are always separated by the character not in s
.