changelog
    
    
    - Mon, 19 Mar 2018 10:59:58 +0200
 
    - by Teemu Piippo <teemu@hecknology.net> [Mon, 19 Mar 2018 10:59:58 +0200] rev 1303
 
    - reworked invert()
 
    - Mon, 19 Mar 2018 10:57:19 +0200
 
    - by Teemu Piippo <teemu@hecknology.net> [Mon, 19 Mar 2018 10:57:19 +0200] rev 1302
 
    - added license headers
 
    - Mon, 19 Mar 2018 10:57:13 +0200
 
    - by Teemu Piippo <teemu@hecknology.net> [Mon, 19 Mar 2018 10:57:13 +0200] rev 1301
 
    - moved inverting code into a new file