public char charBefore(int n) {
        return string.charAt(start-1-n);
    }
Returns the character n places (0 base) before the start of the value substring into the superstring.
For example charBefore(0) returns the first character before the start of the substring

@return the char n planes before the start of the substring
@throws IndexOutOfBoundsException if the string does not have a character at this position
container-search/src/main/java/com/yahoo/prelude/query/Substring.java