signature POLYUNIFICATION =
sig
   datatype (''variable, ''functionSymbol) term =
       Function of ''functionSymbol * (''variable, ''functionSymbol) term list 
     | Variable of ''variable
   type (''variable, ''functionSymbol) equation = 
      (''variable, ''functionSymbol) term * (''variable, ''functionSymbol) term
   type (''variable, ''functionSymbol) binding = 
      ''variable * (''variable, ''functionSymbol) term
   type (''variable, ''functionSymbol) substitution = 
      (''variable, ''functionSymbol) binding list
   datatype equationOutcome = HaltWithFailure | NoMatch | Success
   val unify : (''variable, ''functionSymbol) equation list -> 
       equationOutcome * (''variable, ''functionSymbol) equation list   
end