int idx;                    /* Row index.  First is 1.  0 used for "none" */
  int idxTop;                 /* Direct descendent highest up on the graph */
  GraphRow *pChild;           /* Child immediately above this node */
  u8 isDup;                   /* True if this is duplicate of a prior entry */
  u8 bDescender;              /* True if riser from bottom of graph to here. */
  i8 iRail;                   /* Which rail this check-in appears on. 0-based.*/
  i8 mergeOut;                /* Merge out to this rail.  -1 if no merge-out */
  u8 mergeIn[GR_MAX_RAIL];    /* Merge in from non-zero rails */
  int aiRiser[GR_MAX_RAIL];   /* Risers from this node to a higher row. */

  int mergeUpto;              /* Draw the mergeOut rail up to this level */
  u32 mergeDown;              /* Draw merge lines up from bottom of graph */

  u32 railInUse;              /* Mask of occupied rails at this row */
};

/* Context while building a graph

  if( pParent->mergeOut<0 ){
    u = pParent->aiRiser[pParent->iRail];
    if( u>0 && u<pChild->idx ){
      /* The thick arrow up to the next primary child of pDesc goes
      ** further up than the thin merge arrow riser, so draw them both
      ** on the same rail. */
      pParent->mergeOut = pParent->iRail*4;
      if( pParent->iRail<pChild->iRail ) pParent->mergeOut += 2;
      pParent->mergeUpto = pChild->idx;
    }else{
      /* The thin merge arrow riser is taller than the thick primary
      ** child riser, so use separate rails. */
      int iTarget = pParent->iRail;
      pParent->mergeOut = findFreeRail(p, pChild->idx, pParent->idx-1,
                                       0, iTarget)*4 + 1;
      pParent->mergeUpto = pChild->idx;
      mask = 1<<(pParent->mergeOut/4);
      for(pLoop=pChild->pNext; pLoop && pLoop->rid!=pParent->rid;
           pLoop=pLoop->pNext){
        pLoop->railInUse |= mask;
      }
    }
  }
  pChild->mergeIn[pParent->mergeOut/4] = (pParent->mergeOut&3)+1;
}


/*
** Compute the complete graph
*/
void graph_finish(GraphContext *p, int omitDescenders){

    for(i=1; i<pRow->nParent; i++){
      int parentRid = pRow->aParent[i];
      pDesc = hashFind(p, parentRid);
      if( pDesc==0 ){
        /* Merge from a node that is off-screen */
        int iMrail = findFreeRail(p, pRow->idx, p->nRow, 0, 0);
        mask = 1<<iMrail;
        pRow->mergeIn[iMrail] = 2;
        pRow->mergeDown |= mask;
        for(pLoop=pRow->pNext; pLoop; pLoop=pLoop->pNext){
          pLoop->railInUse |= mask;
        }
      }else{
        /* Merge from an on-screen node */
        createMergeRiser(p, pDesc, pRow);

  /*
  ** Find the maximum rail number.
  */
  p->mxRail = 0;
  for(pRow=p->pFirst; pRow; pRow=pRow->pNext){
    if( pRow->iRail>p->mxRail ) p->mxRail = pRow->iRail;
    if( pRow->mergeOut/4>p->mxRail ) p->mxRail = pRow->mergeOut/4;
    while( p->mxRail<GR_MAX_RAIL && pRow->mergeDown>((1<<(p->mxRail+1))-1) ){
      p->mxRail++;
    }
  }
}