feature request: get available image comparison metrics

I wish there was a way to programatically get the image comparison metrics.

Example:

$ compare -list metric
AE
Fuzz
MAE
MEPP
MSE
NCC
PAE
PSNR
RMSE

$ gm compare -list metric
gm compare: Unrecognized option (-list).

$ compare --version
Version: ImageMagick 6.7.7-10 2013-02-25 Q16 http://www.imagemagick.org
Copyright: Copyright (C) 1999-2012 ImageMagick Studio LLC
Features: OpenMP

$ gm compare -version
GraphicsMagick 1.3.16 2012-06-23 Q8 http://www.GraphicsMagick.org/
Copyright (C) 2002-2012 GraphicsMagick Group.
Additional copyrights and licenses apply to this software.
See http://www.GraphicsMagick.org/www/Copyright.html for details.
...



Steve

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