code: function buildProofChain(later_mci, earlier_mci, unit, arrBalls, onDone){
  	if (earlier_mci === null)
  		throw Error("earlier_mci=null, unit="+unit);
  	if (later_mci === earlier_mci)
  		return buildLastMileOfProofChain(earlier_mci, unit, arrBalls, onDone);
  	buildProofChainOnMc(later_mci, earlier_mci, arrBalls, function(){
  		buildLastMileOfProofChain(earlier_mci, unit, arrBalls, onDone);
  	});
  }
docs: unit's MC index is earlier_mci