def control_node(self, step, package = None):
    '''
    Return a node for the control file given step of this package or optionally another package.
    '''
    if not package:
        package = self.worch.package
    filename = '%s_%s' % (package, step)
    path = self.worch.format('{control_dir}/{filename}', filename=filename)
    return self.path.find_or_declare(path)