A definition may contain the following members:

        - using: a dictionary of variables available for substitution
        - replace: a list of replacement definitions.

def from_definition(cls, defn, names={}):
        """
        A definition may contain the following members:

        - using: a dictionary of variables available for substitution
        - replace: a list of replacement definitions.
        """
        repls = map(Repl.from_defn, defn.get('replace', []))
        self = cls(repls)
        vars(self).update(names)
        vars(self).update(defn.get('using', {}))
        return self