fix(library/init/data/persistenthashmap/basic): isUnaryNode

library/init/data/persistenthashmap/basic.lean

@@ -165,8 +165,9 @@ partial def isUnaryEntries (a : Array (Entry α β (Node α β))) : Nat → Opti
 def isUnaryNode : Node α β → Option (α × β)
 | (Node.entries entries)         := isUnaryEntries entries 0 none
 | (Node.collision keys vals heq) :=
-  if h : 0 < keys.size then
-    some (keys.fget ⟨0, h⟩, vals.fget ⟨0, heq ▸ h⟩)
+  if h : 1 = keys.size then
+    have 0 < keys.size from h ▸ (Nat.zeroLtSucc _);
+    some (keys.fget ⟨0, this⟩, vals.fget ⟨0, heq ▸ this⟩)
   else
     none