This paper reports on an approachfor improvingthe understandingof old programs which have become very complex due to numerous extensions. We have adapted partial evaluation techniques for program understanding. Thesetechniques mainly use propagationthrough statementsand simplificationsof statements. We focus here on the automaticinterproceduralanalysis and we specify both tasks (propagationand simplification)for call-statements,in termsof inferencerules with notations taken from the formal specificationlanguagesB and VDM. We describe how we have implementedthat interproceduralanalysis in a tool, and howit can be used to improveprogramunderstanding.The difficultyofthat analysis is due to the lack of well defined interproceduralmechanismsand the complexityof visibility rules in Fortran.