> (a) X ::= C C ~ 'cc' B B ~ 'bb' A A ~ 'a' :discard ~ A
>
> Forbidden. <A> is required to be a lexeme since it is a discard symbol,
Oh! You are looking at discard symbols as a kind of the same thing as
lexemes, with lexemes the visible part, and discards hidden (*).
But from the perspective of the L0 both are at the top.
(And the same rules about RHS apply).
> and cannot appear on the RHS of an L0 rule.
>
> (b) X ::= C C ~ 'cc' B ~ 'bb' A A ~ 'a' :discard ~ A
>
> <B> is *not* unreachable. It is an L0 lexeme, since it is on an L0 LHS but
> not on any L0 RHS. It might be helpful to imagine that, in L0, the lexemes
> are what the start symbol is in G1. And, of course, start symbols are
> reachable by definition.
Right. And a d'oh moment for me.
> Re (6). When the lexer finds discard and non-discard symbols at the same
> location, the discard symbols are ignored. Discarded symbols are not
> passed up to G1 -- they're discarded!
And that nicely fits in with (*) above.
Ok. I believe I understand this quite a bit better now.
Thank you.
--
See you,
Andreas Kupries <[email protected]>
<http://core.tcl.tk/akupries/>
Developer @ SUSE
Tcl'2017, Oct 16-20, Houston, TX, USA. http://www.tcl.tk/community/tcl2017/
EuroTcl 2017, Jul 8-9, Berlin/DE, http://www.eurotcl.tcl3d.org/
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