🚀 go-pugleaf

I very glad to see some real discussion of errors in Prolog.
This topic is important and I have not yet seen any principled and
acceptable solution.

However, it is worth noting that the question is not as simple as it seems.
Consider for example is.

Now
	X is fred
should clearly generate an error message :-).  But at least once in my career
I desperately wanted it not to.  I wanted to use is to test whether an 
expression was a valid arithmetic expression.  If is had failed silently
then I would have had a nice portable clean way of testing this.  As it was I
had to (sigh) program it all in myself and worry about what was allowable on
different systems and machines.

AS for 
	2 is Y+3
there are number of systems quite able to deduce that Y is 1.  This is the
right way to do it! Not an error message.  But I guess if we must standardize
to obsolete lanagugaes then we have to live in the past:-).

So come on some genius out there give us a good clean solution to this 
problem of errors.

	John G. Cleary
	Department of Computer Science
	University of Calgary
	2500 University Drive 
	N.W. Calgary
	Alberta T2N 1N4
	Canada

	cleary@cpsc.UCalgary.ca
	Phone: (403)282-5711 or (403)220-6087