PyRFLX: prev_field does not return correct predecessor field

[rssen]

Suppose the following Spec:

type Message is
   message
      F1 : F1
         then F2
            if <condition>,
         then F3
            if <other condition>;
      F2 : F2
         then FCS;
      F3 : F3;
      FCS : FCS;
   end message;

The prev_field function is not able to correctly return the predecessor of FCS, because it only checks, whether the condition of an incoming edge can be evaluated to true. It than assumes that the source field of the edge is the predecessor of the given field. As edges form F2 and F3 to FCS both have the condition true, the field which is evaluated first is assumed to be the predecessor of FCS (but not necessarily the right one). The same applies for the _get_length_unchecked function.
To solve this, i would introduce a check, if the assumed predecessor field has been set.