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.