tree insert( key, t )
     typekey key;
     tree t;
     {
     if( t==NULL ) t = NewNode( key, NULL, NULL );
     else if( t->k == key )
          Error; /*** key already in table ***/
     else if( t->k < key ) t->right = insert( key, t->right );
          else             t->left  = insert( key, t->left );
     return( t );
     }