Function EMPTY_ALT

Convenience used to express an empty alternative in an OR (alternation). can be used to more clearly describe the intent in a case of empty alternation.

For example:

1. without using EMPTY_ALT:

  this.OR([
    {ALT: () => {
      this.CONSUME1(OneTok)
      return "1"
    }},
    {ALT: () => {
      this.CONSUME1(TwoTok)
      return "2"
    }},
    // implicitly empty because there are no invoked grammar
    // rules (OR/MANY/CONSUME...) inside this alternative.
    {ALT: () => {
      return "666"
    }},
  ])
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2. using EMPTY_ALT:

  this.OR([
    {ALT: () => {
      this.CONSUME1(OneTok)
      return "1"
    }},
    {ALT: () => {
      this.CONSUME1(TwoTok)
      return "2"
    }},
    // explicitly empty, clearer intent
    {ALT: EMPTY_ALT("666")},
  ])
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