private Item intern(Item item) {
   if (item.sameSymbol(ANY.getSymbol())) return ANY;
   if (item.sameSymbol(VAR.getSymbol())) return VAR;
   if (item.sameSymbol(TERM.getSymbol())) return TERM;
   if (item.sameSymbol(URI.getSymbol())) return URI;
   if (item.sameSymbol(LITERAL.getSymbol())) return LITERAL;
   if (item.sameSymbol(BNODE.getSymbol())) return BNODE;
   return item;
 }