[GM-help] understanding Magick::ImageType with indexed images

Hi

I'm trying to read indexed images and having some trouble identifying
them. Reading the documentation, it seems to me that I can find which
ones are indexed by checking Magick::ClassType (PseudoClass vs
DirectClass) with classType(). Then I could use type(). From the list
here [1],  it seems that indexed images would have to either be
PaletteType or PaletteMatteType. However, I'm getting some indexed
images whose type returns BilevelType and GrayscaleType. How can that
be? And how can I distinguish between an indexed image with and
without opacity values?

On a related note, is it possible to get a string from one of the
Magick::ImageType? I would like my program to print error message with
a bit more info.

Thanks in advance,
Carnë

[1] http://www.graphicsmagick.org/Magick++/Image.html#type