public static <T> String mkString(Collection<T> collection, String start, String sep, String end) {
        return collection.stream()
            .map(T::toString)
            .collect(Collectors.joining(sep, start, end));
     }
Returns a String containing the string representation of all elements from
the given collection, using a start string, separator strings, and an end string.

@param collection The collection
@param start The start string
@param sep The separator string
@param end The end string
@param <T> The element type
@return A string: start + elem(0) + sep + ... + elem(N) + end
vespajlib/src/main/java/com/yahoo/collections/CollectionUtil.java