Usage:
Char is charAt(Source, Pos)

Returns a char from the string Source at the position Pos (zero based). Fails if there is no such symbol.

Actually this function returns not a character but a string with length one. (Such strings we use to represent characters in Strawberry Prolog.)

Source     the source string
Pos        the zero based position
 

Examples:
?- Char is charAt("Hello", 1),
    write(Char). % e