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