Conventions (ref)

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EBNF

We often use EBNF notation of our own.

answers =
  { '42' }

means that answers are a '42' repeated at least once.

name = 'Douglas' [ 'Noel' ] 'Adams'

means that name is either Douglas Adams or Douglas Noel Adams. So the middle name is optional.

character =
    arthur
  | ford
  | trillian
  | zephod

means that character is any of the specified characters.