tree delete( pq )
	tree pq;
	{
	if ( pq == NULL ) Error /*** deletion on an empty queue ***/;
	else if ( pq->left == NULL )	return( NULL );
	else if ( pq->left->left == NULL ) {
		pq->left = pq->right;
		pq->right = NULL;
		}
	else	pq->left = delete( pq->left );
	return( pq );
	};