**Added:**

* Environment variable ``$COMPLETION_MODE`` controls kind of TAB completion used with prompt-toolkit shell.
  ``default``, the default, retains prior Xonsh behavior: first TAB displays the common prefix of matching completions,
  next TAB selects the first or next available completion.
  ``menu-complete`` enables TAB behavior like ``readline`` command ``menu-complete``.  First TAB selects the first matching 
  completion, subsequent TABs cycle through available completions till the last one.  Next TAB after that displays
  the common prefix, then the cycle repeats.

**Changed:**

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**Deprecated:**

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**Removed:**

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**Fixed:**

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**Security:**

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