..
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Ran 2 tests in 0.003s

OK
(command.Simple
  blame_tok: 
    (Token
      id: Id.Lit_Chars
      length: 4
      col: 0
      line: 
        (SourceLine
          line_num: 1
          content: "echo _{a,b}_"
          src: (source.MainFile path:"<cmd_eval_test.py>")
        )
    )
  more_env: []
  words: [
    (CompoundWord parts:[...0x7f9950db8738])
    (word.BracedTree
      parts: [
        (Token id:Id.Lit_Chars length:1 col:5 line:...0x7f9950dcc260)
        (word_part.BracedTuple
          words: [
            (CompoundWord
              parts: [(Token id:Id.Lit_Chars length:1 col:7 line:...0x7f9950dcc260)]
            )
            (CompoundWord
              parts: [(Token id:Id.Lit_Chars length:1 col:9 line:...0x7f9950dcc260)]
            )
          ]
        )
        (Token id:Id.Lit_Chars length:1 col:11 line:...0x7f9950dcc260)
      ]
    )
  ]
  do_fork: T
)
[(Piece s:"" quoted:F do_split:T)]
[(Piece s:xxx quoted:F do_split:T)]
[(Piece s:default quoted:F do_split:T)]
[(Piece s:xxx quoted:F do_split:T)]