Function str_at 

pub fn str_at(s: &SmtString, i: i32) -> SmtString

Character at index i in s, converted to a single-character string

• Characters are indexed from 0 to s.len - 1.
• Return the empty string if i is not in this range.

Example

use aws_smt_strings::smt_strings::*;

let s = SmtString::from("abcd");

assert_eq!(str_at(&s, 2), SmtString::from('c'));
assert_eq!(str_at(&s, 4), EMPTY);