LilyPond concept glossary?

[Mark Polesky]

It would be nice to have some central place that explains some
"internals" concepts. Here are examples of things that a new developer
might have to ask about, or perhaps spend a long time disentangling:

grob
prob
smob
output-def
callback
simple-closure

It would be nice to have a LilyPond-specific glossary, that users could
also use. For example, a user might get "glyph" and "stencil" confused.
I still get "command" "keyword" "identifier" and "variable" confused.
I still don't know the difference between "parser" and "lexer". You get
the idea.

Any thoughts?
- Mark