def _evaluate_f(self, n, xdata, p=None):
<EOL>if   p is None and self.results is not None: p = self.results[<NUM_LIT:0>]<EOL>elif p is None and self.results is None:     p = self._pguess<EOL>args = (xdata,) + tuple(p)<EOL>return self.f[n](*args)<EOL>
Evaluates a single function n for arbitrary xdata and p tuple.

p=None means use the fit results