Consider the following situation:

1. I register package 'p-0.1-inplace' instantiated with 'P = base:Data.Map'
2. I register package 'p-0.1-inplace' instantiated with 'P = base:Control.Monad'

Even though the IPID of these two packages is the same, we clearly want to keep both of them in the database. This thus motivates the following lines in ghc-pkg

     removes = [ RemovePackage p
               | not multi_instance,
                 p <- packages db_to_operate_on,
                 mungedId p == mungedId pkg,
                 -- Only remove things that were instantiated the same way!
                 instantiatedWith p == instantiatedWith pkg ]

OK... now consider the following situation

1. I register package 'p-0.1-inplace' instantiated with 'P = base:Data.Map'
2. I register package 'p-0.1-inplace', instantiated with nothing (I have edited 'p' such that it has no more signatures)

So... ghc-pkg is going to keep both of these packages (per the logic above). But is that really what we want? Now the package database is in a half-consistent state, where the instances of 'p-0.1-inplace' are not consistent with the number of holes they are supposed to have. So let us suggest an invariant:

INVARIANT: All packages which have the same IPID in a package database are self-consistent with each other.

OK, so given this invariant, what do we have to do in step 2? It would seem that when we get a prospective package to register, we must *find all packages which are incompatible with it*, and remove them from the package database. So, given a package with a set of holes H, we must remove all packages with holes H' such that H = H'

I'm not completely convinced this is a good idea. Usually ghc-pkg only removes a single package in response to a new registration, and this behavior could result in lots of packages getting unregistered, more than you might expect. On the other hand, if you think of the package database in a more hierarchical manner, it makes sense that changing the root registration would invalidate all the children.