code: function prepareLinkProofs(arrUnits, callbacks){
  	if (!ValidationUtils.isNonemptyArray(arrUnits))
  		return callbacks.ifError("no units array");
  	if (arrUnits.length === 1)
  		return callbacks.ifError("chain of one element");
  	var arrChain = [];
  	async.forEachOfSeries(
  		arrUnits,
  		function(unit, i, cb){
  			if (i === 0)
  				return cb();
  			createLinkProof(arrUnits[i-1], arrUnits[i], arrChain, cb);
  		},
  		function(err){
  			if (err)
  				return callbacks.ifError(err);
  			callbacks.ifOk(arrChain);
  		}
  	);
  }
docs: arrUnits sorted in reverse chronological order