tree insert( key, t, lev )
     typekey key[];
     tree t;
     int lev;
     {
     int i;
     if ( t==NULL ) t = NewNode( key );

     else {    for ( i=0; i<K && key[i]==t->k[i]; i++ );
          if ( i==K )    Error /*** Key already in table ***/;
          else if ( key[lev] > t->k[lev] )
               t->right = insert( key, t->right, (lev+1)%K );
          else t->left  = insert( key, t->left, (lev+1)%K );
          }
     return( t );
     };