Cleaned up some dumb local libraries.

Schema/src/util/Asserts.cs

@@ -152,21 +152,15 @@ public static bool Equal(
         string? message = null)
       => Equal<string>(expected, actual, message);
 
-    public static bool IsA<TExpected>(object? instance, string? message = null)
-      => IsA(instance, typeof(TExpected), message);
-
-    public static bool IsA(
-        object? instance,
-        Type expected,
-        string? message = null)
-      => Nonnull(instance, message) &&
-         Equal(instance!.GetType(), expected, message);
-
     public static TExpected AsA<TExpected>(
         object? instance,
         string? message = null) {
-      IsA<TExpected>(instance, message);
-      return (TExpected) instance!;
+      if (instance is TExpected expected) {
+        return expected;
+      }
+
+      Asserts.Fail(message);
+      return default!;
     }
 
     public static T Assert<T>(T? value) where T : notnull {

Schema/src/util/enumerables/EnumerableExtensions.cs

@@ -2,6 +2,10 @@
 
 namespace schema.util.enumerables {
   public static class EnumerableExtensions {
+    public static IEnumerable<T> Yield<T>(this T value) {
+      yield return value;
+    }
+
     public static IEnumerable<T> Resized<T>(
         this IEnumerable<T>? enumerable,
         int length) where T : new() {