def fitfcn(self, p):
raise NotImplementedError("<STR_LIT>")<EOL>
Compute fit function fit for parameters ``p``.

        Results are returned in a 1-dimensional array the
        same length as (and corresponding to) the fit data
        returned by ``self.builddata(data)``.

        If marginalization is supported, ``fitfcn`` must work
        with or without the marginalized parameters.

        Args:
            p: Dictionary of parameter values.