#define K 3
    /*** Split the largest, merge the two smallest ***/
    tree checkrots( t )
    tree t;

    { tree R[K], P[K+1];
      int i, j, maxwt, minwt;

      if( wt(t) <= K  ) return( t );
      if( wt( t->left ) < wt( t-> right ) )
           { R[0] = t;  R[1] = t->right;  P[0] = t->left;
             P[1] = R[1]->left;  P[2] = R[1]->right; }
      else { R[0] = t->left;  R[1] = t;  P[0] = R[0]->left;
             P[1] = R[0]->right;  P[2] = t->right; }

      /*** Expand ***/
      for( i=2; i<K; i++ ) {
          for( maxwt=0,j=1; j<=i; j++ )
              if( wt(P[maxwt]) < wt(P[j]) ) maxwt = j;
          for( j=i-1; j >= maxwt; j-- )
              { P[j+2] = P[j+1];  R[j+1] = R[j]; }
          R[maxwt] = P[maxwt];
          P[maxwt] = R[maxwt]->left;  P[maxwt+1] = R[maxwt]->right;
          }

      /*** Merge the two smallest neighbours ***/
      for( i=K; i>0; i-- ) {
          for( minwt=0,j=1; j<i; j++ )
              if( wt(P[minwt])+wt(P[minwt+1]) > wt(P[j])+wt(P[j+1]) )
                  minwt = j;
          R[minwt]->left = P[minwt];  R[minwt]->right = P[minwt+1];
          R[minwt]->weight = wt(P[minwt]) + wt(P[minwt+1]);
          P[minwt] = R[minwt];
          for( j=minwt+1; j<i; j++ )
              { R[j-1] = R[j];  P[j] = P[j+1]; }
          }
      return( P[0] );
    }