..
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Ran 2 tests in 0.003s

OK
(command.Simple
  words: [
    (compound_word
      parts: [
        (Token
          id: Id.Lit_Chars
          col: 0
          length: 4
          span_id: 0
          line: 
            (SourceLine
              line_num: 1
              content: 'echo _{a,b}_'
              src: (source.MainFile path:'<cmd_eval_test.py>')
            )
          tval: echo
        )
      ]
    )
    (word.BracedTree
      parts: [
        (Token
          id: Id.Lit_Underscore
          col: 5
          length: 1
          span_id: 2
          line: 
            (SourceLine
              line_num: 1
              content: 'echo _{a,b}_'
              src: (source.MainFile path:'<cmd_eval_test.py>')
            )
          tval: _
        )
        (word_part.BracedTuple
          words: [
            (compound_word
              parts: [
                (Token
                  id: Id.Lit_Chars
                  col: 7
                  length: 1
                  span_id: 4
                  line: 
                    (SourceLine
                      line_num: 1
                      content: 'echo _{a,b}_'
                      src: (source.MainFile path:'<cmd_eval_test.py>')
                    )
                  tval: a
                )
              ]
            )
            (compound_word
              parts: [
                (Token
                  id: Id.Lit_Chars
                  col: 9
                  length: 1
                  span_id: 6
                  line: 
                    (SourceLine
                      line_num: 1
                      content: 'echo _{a,b}_'
                      src: (source.MainFile path:'<cmd_eval_test.py>')
                    )
                  tval: b
                )
              ]
            )
          ]
          spids: []
        )
        (Token
          id: Id.Lit_Underscore
          col: 11
          length: 1
          span_id: 8
          line: 
            (SourceLine
              line_num: 1
              content: 'echo _{a,b}_'
              src: (source.MainFile path:'<cmd_eval_test.py>')
            )
          tval: _
        )
      ]
    )
  ]
  redirects: []
  more_env: []
  do_fork: T
  spids: []
)
[(part_value.String s:'' quoted:F do_split:T)]
[(part_value.String s:xxx quoted:F do_split:T)]
[(part_value.String s:default quoted:F do_split:T)]
[(part_value.String s:xxx quoted:F do_split:T)]