<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def crossvalidate(self, foldsfile):
 """Train & Test using cross validation, testfile is a file that contains the filenames of all the folds!"""

        options = "-F " + self.format + " " +  self.timbloptions + " -t cross_validate"
        print("Instantiating Timbl API : " + options,file=stderr)
        if sys.version < '3':
            self.api = timblapi.TimblAPI(b(options), b"")
        else:
            self.api = timblapi.TimblAPI(options, "")
        if self.debug:
            print("Enabling debug for timblapi",file=stderr)
            self.api.enableDebug()
        print("Calling Timbl Test : " + options,file=stderr)
        if sys.version < '3':
            self.api.test(b(foldsfile),b'',b'')
        else:
            self.api.test(u(foldsfile),'','')
        a = self.api.getAccuracy()
        del self.api
        return a