(VAR X X1 X2 X3)
(RULES 
a__f(a,X,X) -> a__f(X,a__b,b)
a__b -> a
mark(f(X1,X2,X3)) -> a__f(X1,mark(X2),X3)
mark(b) -> a__b
mark(a) -> a
a__f(X1,X2,X3) -> f(X1,X2,X3)
a__b -> b
)