public static <T> ToString<T> joining(
            CharSequence separator,
            ToString<T> first,
            ToString<T> second) {
        return joining(separator, "", "", first, second);
    }
Returns an expression that represents the result of the two specified
expressions joined together with a separator between them, forming a
single string.

@param separator  the separator (may be empty but never {@code null})
@param first      the first expression
@param second     the second expression
@param <T>        the input entity type
@return           the joined expression
runtime-parent/runtime-compute/src/main/java/com/speedment/runtime/compute/expression/Expressions.java