src/main/java/org/organicdesign/fp/StaticImports.java
@SafeVarargs
    static public <T> MutableRrbt<T> mutableRrb(T... items) {
        if ( (items == null) || (items.length < 1) ) { return RrbTree.emptyMutable(); }
        return RrbTree.<T>emptyMutable()
                .concat(Arrays.asList(items));
    }
@SafeVarargs
    static public <T> MutableRrbt<T> mutableRrb(T... items) {
        if ( (items == null) || (items.length < 1) ) { return RrbTree.emptyMutable(); }
        return RrbTree.<T>emptyMutable()
                .concat(Arrays.asList(items));
    }
Returns a mutable RRB Tree {@link MutableRrbt} of the given items.
The RRB Tree is a list-type data structure that supports random inserts, split, and join
(the PersistentVector does not).  The mutable RRB Tree append() method is only about half
as fast as the PersistentVector method of the same name.  If you build it entirely with random
inserts, then the RRB tree get() method may be about 5x slower.  Otherwise, performance
is about the same.
This data definition method is one of the few methods in this project that support varargs.