public String toTableString() {
		StringBuilder buf = new StringBuilder();
		for (T[] bucket : buckets) {
			if ( bucket==null ) {
				buf.append("null\n");
				continue;
			}
			buf.append('[');
			boolean first = true;
			for (T o : bucket) {
				if ( first ) first=false;
				else buf.append(" ");
				if ( o==null ) buf.append("_");
				else buf.append(o.toString());
			}
			buf.append("]\n");
		}
		return buf.toString();
	}
Add {@code o} to set if not there; return existing value if already
there. This method performs the same operation as {@link #add} aside from
the return value.
runtime/Java/src/org/antlr/v4/runtime/misc/Array2DHashSet.java