(VAR a s t u)
(RULES 

circ(cons(a,s),t) -> cons(msubst(a,t),circ(s,t))

circ(cons(lift,s),cons(a,t)) -> cons(a,circ(s,t))

circ(cons(lift,s),cons(lift,t)) -> cons(lift,circ(s,t))

circ(circ(s,t),u) -> circ(s,circ(t,u))

circ(s,id) -> s

circ(id,s) -> s

circ(cons(lift,s),circ(cons(lift,t),u)) ->
circ(cons(lift,circ(s,t)),u)

subst(a,id) -> a

msubst(a,id) -> a

msubst(msubst(a,s),t) -> msubst(a,circ(s,t)))


(COMMENT Contribution by Eduardo Bonelli)