(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
)