Being able to sub 'a' and 'an' would be a useful feature for tailoring and other descriptions. The use case for this is most apparent when dealing with %color.
For example 'A red baseball cap' versus 'An orange baseball cap'. When using %color, orange or red isn't necessarily known to the tailor, so you could end up with something being 'An red baseball cap' or 'A orange baseball cap.' due to various circumstances.
