production syntax: difference syntax isn't described

[vyivel]

"Production Syntax" section describes the syntax of parenthesization, quantification etc, but doesn't seem to mention the syntax of difference/subtraction/exception (term - string, as seen in e.g. plain-scalar-first-character rule). Looks like an oversight.

[GrahamHannington]

Another example (relating to issue #319):

[34] ns-char ::=
  nb-char - s-white

Reference

• Stack Overflow: How to represent negation in BNF?

[Thom1729]

Off the top of my head, I believe that a - b can always be interpreted [lookahead ≠ b] a. Of course, in cases where a and b are both effectively character classes, then you can equivalently interpret it as set subtraction, and this is surely the intuition behind the operator.