node.set_defaults(a=b...) yields a new calculation node identical to the given node except
        with the default values matching the given key-value pairs. Arguments are collapsed left-to
        right with later arguments overwriting earlier arguments.

def set_defaults(self, *args, **kwargs):
        '''
        node.set_defaults(a=b...) yields a new calculation node identical to the given node except
        with the default values matching the given key-value pairs. Arguments are collapsed left-to
        right with later arguments overwriting earlier arguments.
        '''
        args = merge(self.defaults, args, kwargs)
        new_cnode = copy.copy(self)
        object.__setattr__(new_cnode, 'defaults', ps.pmap(args))
        return new_cnode