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;