_CP_ovfly_subs is equivalent to the result[expression1, expression2] = string1 BASIC statement.
int _CP_ovly_subs(CPSTR** result, CPSTR* string1, int expression1, int expression2)
This function returns -1 if an error occurs. The error code is contained in _CP_errno.
/* Prints "hicdef" */
CPSTR * s = _CP_mkstr("abcdef");
CPSTR * u = _CP_mkstr("hi");
_CP_ovly_subs(&s,u,2,1);
_CP_print(s);