How to find the all userTask elements before the specified userTask element

I have a problem when I do the reject operation on a userTask element, as shown in the picture below: I want to reject current userTask to specified userTask element. The problem is that how to find all userTask elements before the current userTask element.
3

This might be difficult, since you have to define first, what before means.

a) visually (x-coordinate)
b) flow-wise

In terms of b), you could solve this by chaining through the incoming properties and then identify bpmn:UserTask elements.