public final Substring substring(int startIndex, int stopIndex, boolean reverse) {
        return Substring.ofExpressions(toExpression(), startIndex, stopIndex, reverse);
    }
Returns a Substring expression of this expression.

@param startIndex the starting index of the string.
@param stopIndex  the ending index of the string.
@param reverse    whether the indexing is should start from end of the string to start.
@return the {@link Substring} function.
smithy-rules-engine/src/main/java/software/amazon/smithy/rulesengine/language/syntax/SyntaxElement.java