Hint Resolve star1.
Hint Resolve plus1.
Hint Resolve plus_star.
Ltac destar H w :=
match type of H with
star _ ?x ?y => destruct H as [ x | w x y _H1 _H2 ];
[ idtac | generalize (S_plus _ _H1 _H2); clear _H1 _H2; intro H ]
| _ => fail "not a star hypothesis"
end.
Section Plus_WF.
Variable X: Set.
Variable R: relation X.
Hypothesis Rwf: well_founded (trans R).

Theorem plus_wf: well_founded (trans (plus R)).
End Plus_WF.