@classmethod<EOL><INDENT>def custom(cls, mu=<NUM_LIT:1.0>, pi=None, W=None, **kwargs):<DEDENT>
gtr = cls(**kwargs)<EOL>gtr.assign_rates(mu=mu, pi=pi, W=W)<EOL>return gtr<EOL>
Create a GTR model by specifying the matrix explicitly

Parameters
----------

 mu : float
    Substitution rate

 W : nxn matrix
    Substitution matrix

 pi : n vector
    Equilibrium frequencies

 **kwargs:
    Key word arguments to be passed

Keyword Args
------------

 alphabet : str
    Specify alphabet when applicable. If the alphabet specification is
    required, but no alphabet is specified, the nucleotide alphabet will be used as
    default.