An upper set in \(P\) for some preorder \((P, \leq)\)
A subset \(U\) of \(P\) satisfying the condition \(p \in U \land p \leq q \implies q \in U\)
Anything you add to the upper set means you have to add everything greater than it.
Example: the possible upper sets of \(Bool\) are \(\{\varnothing, \{true\}, \{true, false\}\}\)