Prepares the needed files.

def __prepare(self, filestem, pos, neg, b):
        """
        Prepares the needed files.
        """
        posFile = open('%s/%s.f' % (self.tmpdir, filestem), 'w')
        negFile = open('%s/%s.n' % (self.tmpdir, filestem), 'w')
        bFile = open('%s/%s.b' % (self.tmpdir, filestem), 'w')

        posFile.write(pos)
        negFile.write(neg)
        bFile.write(b)

        posFile.close()
        negFile.close()
        bFile.close()