Find the first element of every key for the representative value.

1. By generated identifiers:

   computer[generate-id()=generate-id(key('computer-by-vendor',vendor)[1])]

   This works because the same node must return the same value from generate-id().

2. By node identity:

   computer[count(.|key('computer-by-vendor',vendor)[1]) = 1]

   This works because a node set cannot contain the same node twice.