An expansion. The first three are all variable expansions. The ModifiedExpansion in particular also keeps track of which operation it is to perform. The Char can be any of -+=? and the Bool says whether it was paired with a : in the case of the first four or doubled in the case of the latter two. This isn't a very good data structure, but I hesitate to add 12 more algebraic types, one for each type of expansion. It would be elegant to use a function parameter here, but then we lose our data-ness and it makes it difficult to be Show. We could use a data class that has functions and is also Show and can be pretty-printed, and this would allow arbitrary generalizability, but do we really want this? It needs to be parsed anyway. The other question is the bash extensions: do we parse for / or should it be an error? Is there a way to prevent it optionally?