By default, arguments can be given only once.
However, arguments with the multiple property set to true can be given multiple times, and each time the argument is given, the new value is appended to a list containing all the values.
An example is the \H child argument.
Internally, multiple is implemented by creating a new level in the abstract syntax tree, and storing each argument separately under a newly generated dummy nodes as in:
AstNode: H
  AstArgument: child
    AstNode: Comment
      AstArgument: content
        AstNode: plaintext
        AstNode: x
    AstNode: Comment
      AstArgument: content
        AstNode: plaintext
        AstNode: x

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