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