function drawMergeLine(x0,y0,x1,y1){
    drawLine(mLine,null,x0,y0,x1,y1);
  }
  function drawCherrypickLine(x0,y0,x1,y1){
    drawLine(cpLine,null,x0,y0,x1,y1);
  }
  /* Draw an arrow representing an in-bound merge from the "rail"-th rail
  ** over to the node of "p".  Make is a checkpoint merge is "isCP" is true */
  function drawMergeArrow(p,rail,isCP){
    var x0 = rail*railPitch + node.w/2;
    if( rail in mergeLines ){
      x0 += mergeLines[rail];
      if( p.r<rail ) x0 += mLine.w;
    }else{
      x0 += (p.r<rail ? -1 : 1)*line.w/2;

    **        If this value is omitted if there are no merge children.
    **   mu:  The id of the row which is the top of the merge-out arrow.
    **        Only exists if "mo" exists.
    **   cu:  Extend the mu merge arrow up to this row as a cherrypick
    **        merge line, if this value exists.
    **    u:  Draw a thick child-line out of the top of this node and up to
    **        the node with an id equal to this value.  0 if it is straight to
    **        the top of the page or just up a little wasy, -1 if there is
    **        no thick-line riser (if the node is a leaf).
    **   sb:  Draw a dotted child-line out of the top of this node up to the
    **        node with the id equal to the value.  This is like "u" except
    **        that the line is dotted instead of solid and has no arrow.
    **        Mnemonic: "Same Branch".
    **    f:  0x01: a leaf node.
    **   au:  An array of integers that define thick-line risers for branches.