code: function createLinkProof(later_unit, earlier_unit, arrChain, cb){
  	storage.readJoint(db, later_unit, {
  		ifNotFound: function(){
  			cb("later unit not found");
  		},
  		ifFound: function(objLaterJoint){
  			var later_mci = objLaterJoint.unit.main_chain_index;
  			arrChain.push(objLaterJoint);
  			storage.readUnitProps(db, objLaterJoint.unit.last_ball_unit, function(objLaterLastBallUnitProps){
  				var later_lb_mci = objLaterLastBallUnitProps.main_chain_index;
  				storage.readJoint(db, earlier_unit, {
  					ifNotFound: function(){
  						cb("earlier unit not found");
  					},
  					ifFound: function(objEarlierJoint){
  						var earlier_mci = objEarlierJoint.unit.main_chain_index;
  						var earlier_unit = objEarlierJoint.unit.unit;
  						if (later_mci < earlier_mci && later_mci !== null && earlier_mci !== null)
  							return cb("not included");
  						if (later_lb_mci >= earlier_mci && earlier_mci !== null){ // was spent when confirmed
  							// includes the ball of earlier unit
  							proofChain.buildProofChain(later_lb_mci + 1, earlier_mci, earlier_unit, arrChain, function(){
  								cb();
  							});
  						}
  						else{ // the output was unconfirmed when spent
  							graph.determineIfIncluded(db, earlier_unit, [later_unit], function(bIncluded){
  								if (!bIncluded)
  									return cb("not included");
  								buildPath(objLaterJoint, objEarlierJoint, arrChain, function(){
  									cb();
  								});
  							});
  						}
  					}
  				});
  			});
  		}
  	});
  }
docs: adds later unit earlier unit is not included in the chain