The greatest lower bound (GLB) of two permissions (perm1, perm2), if it exists.
The GLB perm3 is a permission such that perm3 <: perm1 and perm3 <: perm2
but also there is no other mutual lower bound perm4 where perm3 <: perm4.
In other words, if perm1 and perm2 represent upper bounds in inference
(i.e., for some inference variable ?X, ?X <: perm1 and ?X <: perm2),
then perm3 combines those two bounds on ?X into a single bound ?X <: perm3
that must also hold (because there is no possible value for ?X that wouldn’t
satisfy it).
The least upper bound (LUB) of two permissions (perm1, perm2).
The LUB perm3 is a permission such that perm1 <: perm3 and perm1 <: perm3
but also there is no other mutual upper bound perm4 where perm4 <: perm3.
In other words, if perm1 and perm2 represent lower bounds in inference
(i.e., for some inference variable ?X, perm1 <: ?X and perm2 <: ?X),
then perm3 combines those two bounds on ?X into a single bound perm3 <: ?X
that must also hold (because there is no possible value for ?X that wouldn’t
satisfy it).