Determining whether a subprogram may have a side effect is very difficult (indeed, undecidable). A subprogram may have any number and nature of call sites. VCs are used to attempt to establish certain properties hold for a given subprogram. The Atlas was also used for one Mercury flight under the Big Joe subprogram. SUBIN makes these patches every time the subprogram is called. The "voodoo doll" subprogram was eventually rendered useless by a character named Zippy. The entry point need not be the first word of the subprogram.