[magick-users] referencing stacked images with conve

[quetzlzacatenango at imagemagick.org]

> How long have you been keeping it from me?

MPR stands for Magick Persistent Registry.  Its been around for about 10
years.  Anthony uses it on ocassion in his web pages.  We know, ImageMagick,
needs more documentation.  Anthony has documented about 90% of what
ImageMagick can do.  He'll get to 100% within the next few yeats.