public static String mkString(Collection<?> collection, String sep) {
        return mkString(collection, "", sep, "");
    }
Returns a String containing the string representation of all elements from
the given collection, separated by the separator string.

@param collection The collection
@param sep The separator string
@return A string: elem(0) + sep + ... + elem(N)
vespajlib/src/main/java/com/yahoo/collections/CollectionUtil.java