1. p : PosetSig
2. a : |p|
3. b : |p|
⊢ uiff(a <p b;strict_part(x,y.x ≤ y;a;b))
{ RepUnfolds ``strict_part set_lt set_blt`` 0 }
⊢ uiff(↑((a (≤b) b) ∧b (¬b(b (≤b) a)));(a ≤ b) ∧ (¬(b ≤ a)))