Name:
  Interpreter

Intent:
  - define the AST of a language and interpret the AST
    (processor for the textual representation of a problem)
  - replace code with data

Structure:
  - sometimes the AST is a composite
  - interpretation is in the base class
    (overridden in subclasses)

Notes:
  - the language should be simple
  - efficiency is not crucial
  - often works with a Visitor

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Name:
  LittleLanguage

Intent:
  - construct the AST of a language from an external representation

Structure:
  - there is tokenizer (scanner)
  - there is a parser
  - in parser there is a method for each non-terminal of the grammar
  - this method can be constructed mechanically from the grammar rule