whyml: treat 'old marks only in user postconditions
whyml: treat 'old marks only in user postconditions

ssaavedra/why3,douglas-larocca/why3,jpdeplaix/why3,ssaavedra/why3,douglas-larocca/why3,jpdeplaix/why3,ssaavedra/why3,jpdeplaix/why3,douglas-larocca/why3