code: public SimpleExpression<Character> charAt(Expression<Integer> i) {
          return Expressions.comparableOperation(Character.class, Ops.CHAR_AT, mixin, i);
      }
docs: Create a {@code this.charAt(i)} expression
  
  <p>Get the character at the given index</p>
  
  @param i zero based index
  @return this.charAt(i)
  @see java.lang.String#charAt(int)