Allow specialization of a highighting group's subgroups

[lenormf]

Hi,

It'd be nice to be able to specialize a few subgroups of a group that's ref'd to another one, in order not to have to ref all the subgroups except those we want changed.

Example:

# Base group that will be ref'd to
addhl -group / group base
addl -group /base group a
addhl -group /base/a …
addl -group /base group b
addhl -group /base/b …

# Extended group
addhl -group / group extended
# "Inherit" from all the subgroups in base
addl -group /extended group a
addhl -group /extended/a ref base/a
addl -group /extended group b
# Specialize this subgroup, instead of using base's b subgroup
addhl -group /extended/b …

The above code shows how things currently work (to my knowledge), which creates a lot of lines when the base highlighter has a lot of subgroups.

What I wish was possible (or something along those lines):

addhl -group / group extended
# "Inherit" from all the subgroups in base
addl -group /extended ref base
# No need to mention the a subgroup, it's already being used thanks to the above line
# Specialize this subgroup, instead of using base's b subgroup
addhl -group /extended/b …

Thanks.