[mercury-users] discriminated union question

[Richard A. O'Keefe]

	:- type pair(T1, T2)
	        --->    T1 - T2.
	
	What is the "-" sign doing here? What does it mean? It doesn't mean subtraction does it?
	
What is it doing?  Just sitting there.  It's a constructor, is all.
	 
What does it mean?  "The thing that ties together a T1 and a T2 to make
a pair(T1,T2)."

Does it mean subtraction?  No.
	
(-)/2 has been the standard pairing operator in Prolog ever since
keysort/2 was added to DEC-10 Prolog >18 years ago.

(And yes, a referee for the new logic programming journal had never heard
of it nor of Mycroft/O'Keefe types either.  Referees, eh?)
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