cplan.discard_defaults(a, b...) yields a new caclulation plan identical to cplan except 
        without default values for any of the given parameter names.

def discard_defaults(self, *args):
        '''
        cplan.discard_defaults(a, b...) yields a new caclulation plan identical to cplan except 
        without default values for any of the given parameter names.
        '''
        # make a copy of this object's nodes with translations...
        nodes = ps.pmap({k:v.discard_defaults(*args) for (k,v) in six.iteritems(self.nodes)})
        # make a new plan with that!
        return Plan(nodes)