Pre-emptive note here: in #40 the ParameterNodes are always assumed to have values that will vary (see how parameter nodes are gathered here).

To avoid the solver accidentally picking up these constant parameters as values that should vary we could either:

• ParameterNodes have another internal is_constant flag, that defaults to False but can be set to True. Then the aforementioned line checks if node.is_parameter and not node.is_constant.
• ConstantNode class like was suggested before (but I think that's overkill since it would be almost identical to ParameterNode).
• Have the do operator edit the DistributionNode's that now depend on the constant node, by moving the now-fixed value from parameters to constant_parameters for each affected node.

• Have the do operator edit the DistributionNode's that now depend on the constant node, by moving the now-fixed value from parameters to constant_parameters for each affected node.

Ooh, we could just remove the node that the do is setting the value of and do this move without needing to add a new node. This feels very neat so I'll do this