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.
GATO