**Added:**

* <news item>

**Changed:**

* xonsh/environ.py: new rule: if default for "registered" environment variable is set to ``DefaultNotGiven``_ 
  (in env.register(), or DEFAULT_VARS) then variable has no default and raises KeyError if it is not defined in 
  environment.  Likewise, ``"var" in __xonsh__.env``_ will return False.
* Changed defaults for ANSICON, TERM and VIRTUAL_ENV to DefaultNotGiven, so code can rationally test whether
  the expected external program has defined these variables.  No need to do this for variables that xonsh
  itself defines.

**Deprecated:**

* <news item>

**Removed:**

* <news item>

**Fixed:**

* Fixed #3703 and #3739, recent code change made it impossible to tell whether a (registered) environment variable
  was missing from environment or present and set to its registered default value. The test for ANSICON was
  failing due to this.

**Security:**

* <news item>