def on_rule(node, compiled_grammar)
      name = process(node.children[0], compiled_grammar)

      if compiled_grammar.has_terminal?(name)
        compiled_grammar.add_error(
          "the rule name #{name.inspect} is already used as a terminal name",
          node.source_line
        )
      end

      if compiled_grammar.has_rule_with_branches?(name)
        compiled_grammar.add_error(
          "the rule #{name.inspect} has already been defined",
          node.source_line
        )

        return
      end

      branches = node.children[1..-1].map do |child|
        process(child, compiled_grammar)
      end

      rule = compiled_grammar.lookup_rule(name)

      rule.branches.concat(branches)
    end
def on_rule(node, compiled_grammar)
      name = process(node.children[0], compiled_grammar)

      if compiled_grammar.has_terminal?(name)
        compiled_grammar.add_error(
          "the rule name #{name.inspect} is already used as a terminal name",
          node.source_line
        )
      end

      if compiled_grammar.has_rule_with_branches?(name)
        compiled_grammar.add_error(
          "the rule #{name.inspect} has already been defined",
          node.source_line
        )

        return
      end

      branches = node.children[1..-1].map do |child|
        process(child, compiled_grammar)
      end

      rule = compiled_grammar.lookup_rule(name)

      rule.branches.concat(branches)
    end
Processes the assignment of a rule.

 @see [#process]