MAYBE

Problem:
 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()

Proof:
 DP Processor:
  DPs:
   a__f#(a(),b(),X) -> mark#(X)
   a__f#(a(),b(),X) -> a__f#(X,X,mark(X))
   mark#(f(X1,X2,X3)) -> mark#(X3)
   mark#(f(X1,X2,X3)) -> a__f#(X1,X2,mark(X3))
   mark#(c()) -> a__c#()
  TRS:
   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()
  Open