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The grammar file of the parser contains the parser rule.
The most common approach is to combine combined in in one grammar file, the lexer and parser grammar.
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Syntax
parser grammar myGrammar;
If you have two differents file and your lexer grammar is defined in another file, you need to define it with the tokenVocab option.
parser grammar MarkupParser;
options { tokenVocab=MarkupLexer; }