Action Graph
Start with the Tree Graph with the strings included as dashed edges. Replace each edge by c_i edges in parallel, one for each Constraints of Motion of the relative joint. The replaced edges carry a wrench related to the constraint it adds.
If the original edge was a branch, only one parallel edge remains a branch, all the others become strings. If the original edge was a string, all the parallel edges become strings.
External forces and moments should be added as strings going from the support link to the link where the force/moment is being applied.
The wrenchs can be constructed by Wrench of a Joint, but at the action graph they should be only refered by a symbol.