the type of strings that type t is a substring of
create ?pos ?len base creates a substring of the base sequence of
len starting at position
base.[pos], base.[pos + 1], ... base.[pos + len - 1]
It is required that:
0 <= pos
0 <= len
pos + len <= length base
It does not copy the characters.
for copying characters from a substring to and from both strings and substrings.
The result of these functions may share data with their input.