Generate arbitrary keyed terms.

prototype/DoubtTests/RangedTerm.swift

@@ -19,9 +19,17 @@ extension UnannotatedTerm: Arbitrary {
 				}
 			}
 		}
+		let keyed: Gen<Term> = Gen.sized { n in
+			Gen<Int>.choose((0, n)).bind { n in
+				sequence((0..<n).map { _ in String.arbitrary.bind { key in UnannotatedTerm.arbitrary.fmap { (key, $0.term) } } }).fmap {
+					Term((), .Keyed(Dictionary(elements: $0)))
+				}
+			}
+		}
 		return Gen.oneOf([
 			leaf,
 			indexed,
+			keyed,
 		]).fmap {
 			UnannotatedTerm(term: $0)
 		}