I think this test demonstrates how absorbing these three materials fails. This is a serious problem. I haven't looked into it at all, but I wanted to submit this test as a pull request so that I can link it to an issue I'll describe. I hope it can be reviewed by someone else in case I'm missing something. @gidden , @rwcarlsen ?
This test shows that it doesn't have to be three materials. It just has to be such that the absorber material and the absorbed material have different quantities and the absorber material has a subset of the materials in the absorbed material.
 This additional expectation shows the way in which the other isotopes are altered incorrectly as well.
it was actually just the ratio, which was completely backwards. I'm alarmed that this hasn't broken more stuff.
merge's katy's fix to absorb
katy found a (deleterious) bug in the material class absorption method. tests have been added that broke the original code base but now pass with her fix. this closes #493.