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From: joel de guzman (isis-tech_at_[hidden])
Date: 2001-06-01 20:55:36

From: "David Abrahams":
> > a | b Union (match a or b. Also referred to as alternatives)
> OK.
> > a & b Intersection (match a and b)
> Haven't been following closely, but what does this mean? Is it the same as
> "ab" in conventional BNF? or perhaps "ab|ba" ?
> > a - b Difference (match a but not b)
> > a ^ b XOR (match a or b, but not both)
> Likewise, what does it mean to match "both"?
> -Dave

See the link to Dr. Dick Botting on XBNF:
How far can BNF stretch. Dr. Botting gave some
insights and proof that Intersections let us define languages
that are not context free:

a & b

Indicates that all the descriptions of a and b must hold
at once (set intersection), not in sequence.

a ^ b

Is somewhat similar to | (set union, alternatives)
with the added restriction that only one (not both)
should hold.

... just your basic boolean algebra truth tables applied to

Joel de Guzman

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