+alias symbol "square" (instance 7) = "ordered set square".
+lemma ss_of_bs:
+ ∀O:ordered_set.∀u,v:O.
+ ∀b:O square.\fst b ∈ [u,v] → \snd b ∈ [u,v] → {[u,v]} square ≝
+ λO:ordered_set.λu,v:O.
+ λb:(O:bishop_set) square.λH1,H2.〈≪\fst b,H1≫,≪\snd b,H2≫〉.
+
+notation < "x \sub \nleq" with precedence 91 for @{'ssbs $x}.
+interpretation "ss_of_bs" 'ssbs x = (ss_of_bs _ _ _ x _ _).
+