Fix simplify_formula occurence_check ( :( )
Fix simplify_formula occurence_check ( :( )

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