make function types with an empty domain subsingletons

this is useful for the base cases of W types when i try those again

closes #23

examples/eta.quox

@@ -1,3 +1,5 @@
+load "misc.quox"
+
 namespace eta {
 
 def0 Π : (A : ★) → (A → ★) → ★ = λ A B ⇒ (x : A) → B x
@@ -10,4 +12,9 @@ def0 box : (A : ★) → (P : [ω.A] → ★) → (e : [ω.A]) →
            P [case1 e return A of {[x] ⇒ x}] → P e =
   λ A P e p ⇒ p
 
+-- not exactly η, but kinda related
+def0 from-false : (A : ★) → (P : (False → A) → ★) → (f : False → A) →
+                  P (λ x ⇒ void A x) → P f =
+  λ A P f p ⇒ p
+
 }