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# ->retrieveByUsername()
 -  returns  if username is invalid
 -  returns  if user is inactive
 -  returns an inactive user when second parameter is false
 -  returns an active user
 -  returns  if user is active and second parameter is false
 -  can be called non-statically
 -  does not throw an exception if username is null
 -  can be called statically (BC)
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