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Ran 2 tests in 0.001s

OK
(SimpleCommand
  words: [
    (CompoundWord parts:[(LiteralPart token:(token id:Lit_Chars val:echo span_id:0))])
    (BracedWordTree
      parts: [
        (LiteralPart token:(token id:Lit_Chars val:_ span_id:2))
        (BracedAltPart
          words: [
            (CompoundWord
              parts: [(LiteralPart token:(token id:Lit_Chars val:a span_id:4))]
            )
            (CompoundWord
              parts: [(LiteralPart token:(token id:Lit_Chars val:b span_id:6))]
            )
          ]
        )
        (LiteralPart token:(token id:Lit_Chars val:_ span_id:8))
      ]
    )
  ]
)
[(StringPartValue s:'' do_split_glob:T)]
[(StringPartValue s:xxx do_split_glob:T)]
[(StringPartValue s:default do_split_glob:T)]
[(StringPartValue s:xxx do_split_glob:T)]