code: function(oStep){
  		var bIsTopLevel = oStep.getParent() instanceof sap.ui.commons.RoadMap;
  		var iIdx = oStep.getParent()[bIsTopLevel ? "indexOfStep" : "indexOfSubStep"](oStep);
  		var iCountInvisible = 0;
  		var aSteps = oStep.getParent()[bIsTopLevel ? "getSteps" : "getSubSteps"]();
  		for (var i = 0; i < iIdx; i++) {
  			if (!aSteps[i].getVisible()) {
  				iCountInvisible++;
  			}
  		}
  		return iIdx + 1 - iCountInvisible;
  	}
docs: Computes how the aria-posinset property should be set for the given step