It would be great if one could affiliate highlights. There are 2 ways that I have thought of that would help me, and possibly others, immensely:
1) Continuous Grouping - allows the affiliation of highlights that follow immediately one after the other. This would allow the use of one color for key words, and another color for important text immediately following that word. In tat way, when you filter for only the key word's highlight color, you can view the key words without explaination, giving a great way to self test.
2) Tagging - allow the custom tagging of highlights, so that the user can view them filtered and grouped as desired