Create TransformedStructure from SNL.

        Args:
            snl (StructureNL): Starting snl

        Returns:
            TransformedStructure

def from_snl(cls, snl):
        """
        Create TransformedStructure from SNL.

        Args:
            snl (StructureNL): Starting snl

        Returns:
            TransformedStructure
        """
        hist = []
        for h in snl.history:
            d = h.description
            d['_snl'] = {'url': h.url, 'name': h.name}
            hist.append(d)
        return cls(snl.structure, history=hist)