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Description:

gkdbnf 0.4.2

GKD-BNF
pip install gkdbnf.
Maybe the most simplest way to write pretty BNF in LaTex.
Use this package with gkdtex:
Usage
\gkd@bnf{
<i, j> :: Integer
<b> :: Boolean
<l> :: Labels

!Instructions! <h> :: !Instr[MinSIL]! = \mathbf{load} \; i %%% \qquad \qquad describe load
| \mathbf{store} \; i
| \mathbf{push} \; \zeta
| \mathbf{pop}
| \mathbf{jump\mbox{-}if} l %%% consume TOS.
| \cdots

<e> ::= <e> ( <e> * ) | atom

}

Syntax
Valid BNF Syntax:
atom ::= NONTERMINAL
| TERMINAL
| TERMINAL2
| '|'

rule ::= '%%%' TERMINAL2
description ::= TERMINAL | TERMINAL2
type ::= TERMINAL | TERMINAL2 | NONTERMINAL

prod ::= description? NONTERMINAL '::' type '=' atom+ rule?
| '|' atom+ rule?

start ::= start NEWLINE
| start prod
| NEWLINE
| prod

Lexer rule by regex:
NEWLINE = [\r\n]+
NONTERMINAL = <.*?>
TERMINAL2 = !.*?!
TERMINAL = \S+

Whitespace tokens are ignored.
Nice Error Report
1: \gkd@usepackage{gkdbnf}
2: \gkd@bnf{
3: <a> ::= a + 1
4:
5: <c> ::= a a
6: Expressions e ::= e
7: }

You get error
SyntaxError: filename runtest/a.tex:
line 6, column 13, NonTerm not match

Then you know you should change Expression e to Expression <e>.