diff --git a/README.md b/README.md
index d3bfb368..a30803cc 100644
--- a/README.md
+++ b/README.md
@@ -27,7 +27,7 @@ Maven dependency:
   <groupId>org.microbean</groupId>
   <artifactId>microbean-lang</artifactId>
   <!-- Always check https://search.maven.org/artifact/org.microbean/microbean-lang for up-to-date available versions. -->
-  <version>0.0.19</version>
+  <version>0.0.20</version>
 </dependency>
 ```
 
diff --git a/lang/src/main/java/org/microbean/lang/Lang.java b/lang/src/main/java/org/microbean/lang/Lang.java
index 4fbde8a9..953e3628 100644
--- a/lang/src/main/java/org/microbean/lang/Lang.java
+++ b/lang/src/main/java/org/microbean/lang/Lang.java
@@ -2127,8 +2127,8 @@ private static final TypeMirror[] typeArray(final Type[] ts) {
   /**
    * Asynchronously and idempotently initializes the {@link Lang} class for use.
    *
-   * <p>This method is automatically called when appropriate, but is {@code public} to support eager initialization use
-   * cases.</p>
+   * <p>This method is automatically called by the internals of this class when appropriate, but is {@code public} to
+   * support eager initialization use cases.</p>
    */
   // Idempotent.
   public static final void initialize() {
@@ -2292,6 +2292,11 @@ public final boolean sameType(final TypeMirror t, final TypeMirror s) {
       return t == s || Lang.sameType(t, s);
     }
 
+    @Override
+    public final boolean subtype(final TypeMirror t, final TypeMirror s) {
+      return t == s || Lang.subtype(t, s);
+    }
+
     @Override
     public final TypeElement typeElement(final CharSequence canonicalName) {
       return Lang.typeElement(canonicalName);
@@ -2466,8 +2471,10 @@ public final void run() {
           options.add("-verbose");
         }
 
-        final DiagnosticLogger diagnosticLogger = new DiagnosticLogger(Locale.getDefault());
-        final StandardJavaFileManager sjfm = jc.getStandardFileManager(diagnosticLogger, Locale.getDefault(), Charset.defaultCharset());
+        final Locale defaultLocale = Locale.getDefault();
+
+        final DiagnosticLogger diagnosticLogger = new DiagnosticLogger(defaultLocale);
+        final StandardJavaFileManager sjfm = jc.getStandardFileManager(diagnosticLogger, defaultLocale, Charset.defaultCharset());
 
         // (Any "loading" is actually performed by, e.g. com.sun.tools.javac.jvm.ClassReader.fillIn(), not reflective
         // machinery. Once a class has been so loaded, com.sun.tools.javac.code.Symtab#getClass(ModuleSymbol, Name) will
@@ -2480,7 +2487,7 @@ public final void run() {
                      List.of("java.lang.annotation.RetentionPolicy"), // arbitrary, but loads the least amount of stuff up front
                      null); // compilation units; null means we aren't actually compiling anything
         task.setProcessors(List.of(new P()));
-        task.setLocale(Locale.getDefault());
+        task.setLocale(defaultLocale);
         task.addModules(additionalRootModuleNames);
 
         if (LOGGER.isLoggable(DEBUG)) {
diff --git a/lang/src/main/java/org/microbean/lang/TypeAndElementSource.java b/lang/src/main/java/org/microbean/lang/TypeAndElementSource.java
index 5a313ffd..4ac5b863 100644
--- a/lang/src/main/java/org/microbean/lang/TypeAndElementSource.java
+++ b/lang/src/main/java/org/microbean/lang/TypeAndElementSource.java
@@ -74,6 +74,8 @@ public DeclaredType declaredType(final TypeElement typeElement,
 
   public boolean sameType(final TypeMirror t, final TypeMirror s);
 
+  public boolean subtype(final TypeMirror t, final TypeMirror s);
+
   public TypeElement typeElement(final CharSequence canonicalName);
 
   // Note that Elements#getTypeElement(ModuleElement, CharSequence), to which this basically ultimately delegates, says
@@ -91,49 +93,48 @@ public DeclaredType declaredType(final TypeElement typeElement,
    */
 
 
-  public default ArrayType arrayType(final Class<?> c) {
-    if (!c.isArray()) {
-      throw new IllegalArgumentException("c: " + c);
-    }
-    return this.arrayTypeOf(this.type(c.getComponentType()));
-  }
-
-  public default ArrayType arrayType(final GenericArrayType g) {
-    return this.arrayTypeOf(this.type(g.getGenericComponentType()));
+  public default ArrayType arrayType(final Type t) {
+    return switch (t) {
+    case null                        -> throw new NullPointerException("t");
+    case Class<?> c when c.isArray() -> this.arrayTypeOf(this.type(c.getComponentType()));
+    case GenericArrayType g          -> this.arrayTypeOf(this.type(g.getGenericComponentType()));
+    default                          -> throw new IllegalArgumentException("t: " + t);
+    };
   }
 
   public default DeclaredType declaredType(final CharSequence canonicalName) {
     return this.declaredType(this.typeElement(canonicalName));
   }
 
-  public default DeclaredType declaredType(final Class<?> c) {
-    if (c.isArray() || c.isPrimitive() || c.isLocalClass() || c.isAnonymousClass()) {
-      throw new IllegalArgumentException("c: " + c);
+  public default DeclaredType declaredType(final Type t) {
+    return switch (t) {
+    case null                -> throw new NullPointerException("t");
+    case Class<?> c when c.isArray() || c.isPrimitive() || c.isLocalClass() || c.isAnonymousClass()
+                             -> throw new IllegalArgumentException("t: " + t);
+    case Class<?> c          -> {
+      final Class<?> ec = c.getEnclosingClass();
+      yield ec == null ? this.declaredType(this.typeElement(c)) : this.declaredType(this.declaredType(ec), this.typeElement(c));
     }
-    final Class<?> ec = c.getEnclosingClass();
-    return this.declaredType(ec == null ? null : this.declaredType(ec), this.typeElement(c));
-  }
-
-  public default DeclaredType declaredType(final ParameterizedType p) {
-    return
-      this.declaredType(switch (p.getOwnerType()) {
-        case null                 -> null;
-        case Class<?> c           -> this.declaredType(c);
-        case ParameterizedType pt -> this.declaredType(pt);
-        default                   -> throw new IllegalArgumentException("p: " + p);
-        },
-        this.typeElement((Class<?>)p.getRawType()),
-        this.typeArray(p.getActualTypeArguments()));
+    case ParameterizedType p -> this.declaredType(switch (p.getOwnerType()) {
+      case null                 -> null;
+      case Class<?> c           -> this.declaredType(c);
+      case ParameterizedType pt -> this.declaredType(pt);
+      default                   -> throw new IllegalArgumentException("t: " + t);
+      },
+      this.typeElement(p.getRawType()),
+      this.typeArray(p.getActualTypeArguments()));
+    default                  -> throw new IllegalArgumentException("t: " + t);
+    };
   }
 
   public default Optional<? extends ConstantDesc> describeConstable(final AnnotatedConstruct a) {
     return switch (a) {
-    case null -> Optional.of(NULL);
-    case Constable c -> c.describeConstable();
+    case null            -> Optional.of(NULL);
+    case Constable c     -> c.describeConstable();
     case ConstantDesc cd -> Optional.of(cd); // future proofing
-    case TypeMirror t -> Optional.empty();
-    case Element e -> Optional.empty();
-    default -> throw new IllegalArgumentException("a: " + a);
+    case TypeMirror t    -> Optional.empty();
+    case Element e       -> Optional.empty();
+    default              -> throw new IllegalArgumentException("a: " + a);
     };
   }
 
@@ -146,16 +147,17 @@ public default PrimitiveType primitiveType(final Class<?> c) {
 
   public default TypeMirror type(final Type t) {
     return switch (t) {
-    case null                                 -> throw new NullPointerException();
-    case Class<?> c when c == void.class      -> this.noType(TypeKind.VOID);
-    case Class<?> c when c.isArray()          -> this.arrayType(c);
-    case Class<?> c when c.isPrimitive()      -> this.primitiveType(c);
-    case Class<?> c                           -> this.declaredType(c);
-    case ParameterizedType p                  -> this.declaredType(p);
-    case GenericArrayType g                   -> this.arrayType(g);
-    case java.lang.reflect.TypeVariable<?> tv -> this.typeVariable(tv);
-    case java.lang.reflect.WildcardType w     -> this.wildcardType(w);
-    default                                   -> throw new IllegalArgumentException("t: " + t);
+    case null                                                     -> throw new NullPointerException();
+    case Class<?> c when c == void.class                          -> this.noType(TypeKind.VOID);
+    case Class<?> c when c.isArray()                              -> this.arrayType(c);
+    case Class<?> c when c.isPrimitive()                          -> this.primitiveType(c);
+    case Class<?> c when c.isLocalClass() || c.isAnonymousClass() -> throw new IllegalArgumentException("t: " + t);
+    case Class<?> c                                               -> this.declaredType(c);
+    case ParameterizedType p                                      -> this.declaredType(p);
+    case GenericArrayType g                                       -> this.arrayType(g);
+    case java.lang.reflect.TypeVariable<?> tv                     -> this.typeVariable(tv);
+    case java.lang.reflect.WildcardType w                         -> this.wildcardType(w);
+    default                                                       -> throw new IllegalArgumentException("t: " + t);
     };
   }
 
@@ -167,11 +169,15 @@ public default TypeMirror[] typeArray(final Type[] ts) {
     return rv;
   }
 
-  public default TypeElement typeElement(final Class<?> c) {
-    if (c.isArray() || c.isPrimitive() || c.isLocalClass() || c.isAnonymousClass()) {
-      throw new IllegalArgumentException("c: " + c);
-    }
-    return this.typeElement(c.getCanonicalName());
+  public default TypeElement typeElement(final Type t) {
+    return switch (t) {
+    case null                -> throw new NullPointerException("t");
+    case Class<?> c when c.isArray() || c.isPrimitive() || c.isLocalClass() || c.isAnonymousClass()
+                             -> throw new IllegalArgumentException("t: " + t);
+    case Class<?> c          -> this.typeElement(c.getCanonicalName());
+    case ParameterizedType p -> this.typeElement(p.getRawType());
+    default                  -> throw new IllegalArgumentException("t: " + t);
+    };
   }
 
   public default WildcardType wildcardType(final java.lang.reflect.WildcardType t) {
diff --git a/lang/src/main/java/org/microbean/lang/visitor/AdaptingVisitor.java b/lang/src/main/java/org/microbean/lang/visitor/AdaptingVisitor.java
index 646c55b9..dbf9fd04 100644
--- a/lang/src/main/java/org/microbean/lang/visitor/AdaptingVisitor.java
+++ b/lang/src/main/java/org/microbean/lang/visitor/AdaptingVisitor.java
@@ -41,7 +41,7 @@
 import static org.microbean.lang.type.Types.asElement;
 
 /**
- * Does something adapting-like.
+ * Does something adapting-like. Looks to be related to type variable resolution.
  *
  * <p>Usage: call {@link #adapt(DeclaredType, DeclaredType)}, not {@link #visit(TypeMirror)}.</p>
  *
@@ -67,6 +67,7 @@ final class AdaptingVisitor extends SimpleTypeVisitor14<Void, TypeMirror> {
 
   private final Types types;
 
+  // The compiler calls this "mapping"; the class is called "Adapting"; I think they are the same thing
   private final Map<DelegatingElement, TypeMirror> mapping;
 
   private final SameTypeVisitor sameTypeVisitor;
@@ -92,6 +93,10 @@ final class AdaptingVisitor extends SimpleTypeVisitor14<Void, TypeMirror> {
     this.to = Objects.requireNonNull(to, "to");
   }
 
+  final void adaptSelf(final DeclaredType target) {
+    this.adapt((DeclaredType)target.asElement().asType(), target);
+  }
+
   final void adapt(final DeclaredType source, final DeclaredType target) {
     this.visitDeclared(source, target);
     final int fromSize = this.from.size();
@@ -103,10 +108,29 @@ final void adapt(final DeclaredType source, final DeclaredType target) {
     }
   }
 
-  final void adaptSelf(final DeclaredType target) {
-    this.adapt((DeclaredType)target.asElement().asType(), target);
+  private final void adaptRecursive(final Collection<? extends TypeMirror> source, final Collection<? extends TypeMirror> target) {
+    if (source.size() == target.size()) {
+      final Iterator<? extends TypeMirror> sourceIterator = source.iterator();
+      final Iterator<? extends TypeMirror> targetIterator = target.iterator();
+      while (sourceIterator.hasNext()) {
+        assert targetIterator.hasNext();
+        this.adaptRecursive(sourceIterator.next(), targetIterator.next());
+      }
+    }
   }
 
+  private final void adaptRecursive(final TypeMirror source, final TypeMirror target) {
+    final TypeMirrorPair pair = new TypeMirrorPair(this.sameTypeVisitor, source, target);
+    if (this.cache.add(pair)) {
+      try {
+        this.visit(source, target);
+      } finally {
+        this.cache.remove(pair);
+      }
+    }
+  }
+
+  // Adapts the component type of source.
   @Override
   public final Void visitArray(final ArrayType source, final TypeMirror target) {
     assert source.getKind() == TypeKind.ARRAY;
@@ -116,6 +140,8 @@ public final Void visitArray(final ArrayType source, final TypeMirror target) {
     return null;
   }
 
+  // Adapts type arguments of source. "Recursive" means something like "go into type parameters etc."
+  // Adapting a type argument takes effect on wildcard-extending type variables only, it looks like.
   @Override
   public final Void visitDeclared(final DeclaredType source, final TypeMirror target) {
     assert source.getKind() == TypeKind.DECLARED;
@@ -135,6 +161,7 @@ public final Void visitTypeVariable(final TypeVariable source, final TypeMirror
       this.from.add(source);
       this.to.add(target);
     } else if (val.getKind() == TypeKind.WILDCARD && target.getKind() == TypeKind.WILDCARD) {
+      // This block is the only place in this entire class where the magic happens.
       final WildcardType valWc = (WildcardType)val;
       final TypeMirror valSuperBound = valWc.getSuperBound();
       final TypeMirror valExtendsBound = valWc.getExtendsBound();
@@ -172,6 +199,7 @@ public final Void visitTypeVariable(final TypeVariable source, final TypeMirror
     return null;
   }
 
+  // Adapts the bounds of a wildcard.
   @Override
   public final Void visitWildcard(final WildcardType source, final TypeMirror target) {
     assert source.getKind() == TypeKind.WILDCARD;
@@ -191,26 +219,5 @@ public final Void visitWildcard(final WildcardType source, final TypeMirror targ
     return null;
   }
 
-  private final void adaptRecursive(final TypeMirror source, final TypeMirror target) {
-    final TypeMirrorPair pair = new TypeMirrorPair(this.sameTypeVisitor, source, target);
-    if (this.cache.add(pair)) {
-      try {
-        this.visit(source, target);
-      } finally {
-        this.cache.remove(pair);
-      }
-    }
-  }
-
-  private final void adaptRecursive(final Collection<? extends TypeMirror> source, final Collection<? extends TypeMirror> target) {
-    if (source.size() == target.size()) {
-      final Iterator<? extends TypeMirror> sourceIterator = source.iterator();
-      final Iterator<? extends TypeMirror> targetIterator = target.iterator();
-      while (sourceIterator.hasNext()) {
-        assert targetIterator.hasNext();
-        this.adaptRecursive(sourceIterator.next(), targetIterator.next());
-      }
-    }
-  }
 
 }
diff --git a/lang/src/main/java/org/microbean/lang/visitor/PrecedesPredicate.java b/lang/src/main/java/org/microbean/lang/visitor/PrecedesPredicate.java
index e58bca73..56b46cf8 100644
--- a/lang/src/main/java/org/microbean/lang/visitor/PrecedesPredicate.java
+++ b/lang/src/main/java/org/microbean/lang/visitor/PrecedesPredicate.java
@@ -61,7 +61,7 @@ public final boolean test(final Element e, final Element f) {
     case DECLARED:
       switch (s.getKind()) {
       case DECLARED:
-        if (this.equality.equals(e, f)) { // in place of ==
+        if (this.equality.equals(e, f)) { // already checked e == f
           // Both are completely interchangeable DeclaredTypes; can't say which comes first.
           return false;
         }
@@ -77,7 +77,7 @@ public final boolean test(final Element e, final Element f) {
     case TYPEVAR:
       switch (s.getKind()) {
       case TYPEVAR:
-        if (this.equality.equals(e, f)) { // in place of ==
+        if (this.equality.equals(e, f)) { // already checked e == f
           // Both are completely interchangeable TypeVariables; can't say which comes first.
           return false;
         }
diff --git a/lang/src/main/java/org/microbean/lang/visitor/SupertypeVisitor.java b/lang/src/main/java/org/microbean/lang/visitor/SupertypeVisitor.java
index 246af148..bd27d313 100644
--- a/lang/src/main/java/org/microbean/lang/visitor/SupertypeVisitor.java
+++ b/lang/src/main/java/org/microbean/lang/visitor/SupertypeVisitor.java
@@ -151,7 +151,15 @@ public final TypeMirror visitDeclared(final DeclaredType t, final Void x) {
   @Override // SimpleTypeVisitor14
   public final TypeMirror visitIntersection(final IntersectionType t, final Void x) {
     assert t.getKind() == TypeKind.INTERSECTION;
-    return t.getBounds().get(0); // TODO: presumes first bound will be the supertype; see https://github.com/openjdk/jdk/blob/jdk-19+25/src/jdk.compiler/share/classes/com/sun/tools/javac/code/Type.java#L1268
+    // Since an intersection type is always ordered from most specialized to least, it follows that the direct supertype
+    // of an intersection type is its first bound. See
+    // https://docs.oracle.com/javase/specs/jls/se22/html/jls-4.html#jls-4.10.2
+    //
+    // TODO: wait, no, this isn't correct at all. The JLS says: "The direct supertypes of an intersection type T1 &
+    // ... & Tn are Ti (1 ≤ i ≤ n)." But javac's Types class sets the supertype_field (singular) field to the first
+    // element of the list. See
+    // https://github.com/openjdk/jdk/blob/jdk-19+25/src/jdk.compiler/share/classes/com/sun/tools/javac/code/Type.java#L1268.
+    return t.getBounds().get(0);
   }
 
   @Override // SimpleTypeVisitor14
diff --git a/lang/src/test/java/org/microbean/lang/TestErase.java b/lang/src/test/java/org/microbean/lang/TestErase.java
index 86dcf5b0..8eeea1ae 100644
--- a/lang/src/test/java/org/microbean/lang/TestErase.java
+++ b/lang/src/test/java/org/microbean/lang/TestErase.java
@@ -151,12 +151,10 @@ public final PrimitiveType primitiveType(final TypeKind k) {
         public boolean sameType(final TypeMirror t, final TypeMirror s) {
           return javacModelTypes.isSameType(t, s);
         }
-        /*
         @Override
-        public final TypeElement typeElement(final CharSequence m, final CharSequence n) {
-          return elements.getTypeElement(elements.getModuleElement(m), n);
+        public boolean subtype(final TypeMirror t, final TypeMirror s) {
+          return javacModelTypes.isSubtype(t, s);
         }
-        */
         @Override
         public final TypeElement typeElement(final CharSequence n) {
           return elements.getTypeElement(n);
diff --git a/lang/src/test/java/org/microbean/lang/TestSameTypeVisitor.java b/lang/src/test/java/org/microbean/lang/TestSameTypeVisitor.java
index 9727fe98..690b6dfe 100644
--- a/lang/src/test/java/org/microbean/lang/TestSameTypeVisitor.java
+++ b/lang/src/test/java/org/microbean/lang/TestSameTypeVisitor.java
@@ -117,12 +117,10 @@ public final PrimitiveType primitiveType(final TypeKind k) {
         public boolean sameType(final TypeMirror t, final TypeMirror s) {
           return javacModelTypes.isSameType(t, s);
         }
-        /*
         @Override
-        public final TypeElement typeElement(final CharSequence m, final CharSequence n) {
-          return elements.getTypeElement(elements.getModuleElement(m), n);
+        public boolean subtype(final TypeMirror t, final TypeMirror s) {
+          return javacModelTypes.isSubtype(t, s);
         }
-        */
         @Override
         public final TypeElement typeElement(final CharSequence n) {
           return elements.getTypeElement(n);
diff --git a/lang/src/test/java/org/microbean/lang/TestSupertypeVisitor.java b/lang/src/test/java/org/microbean/lang/TestSupertypeVisitor.java
index 108db893..fbbcda85 100644
--- a/lang/src/test/java/org/microbean/lang/TestSupertypeVisitor.java
+++ b/lang/src/test/java/org/microbean/lang/TestSupertypeVisitor.java
@@ -137,12 +137,10 @@ public final PrimitiveType primitiveType(final TypeKind k) {
         public boolean sameType(final TypeMirror t, final TypeMirror s) {
           return javacModelTypes.isSameType(t, s);
         }
-        /*
         @Override
-        public final TypeElement typeElement(final CharSequence m, final CharSequence n) {
-          return elements.getTypeElement(elements.getModuleElement(m), n);
+        public boolean subtype(final TypeMirror t, final TypeMirror s) {
+          return javacModelTypes.isSubtype(t, s);
         }
-        */
         @Override
         public final TypeElement typeElement(final CharSequence n) {
           return elements.getTypeElement(n);
diff --git a/lang/src/test/java/org/microbean/lang/TestTypeClosure.java b/lang/src/test/java/org/microbean/lang/TestTypeClosure.java
index 865c21d3..ee9e09cd 100644
--- a/lang/src/test/java/org/microbean/lang/TestTypeClosure.java
+++ b/lang/src/test/java/org/microbean/lang/TestTypeClosure.java
@@ -132,12 +132,10 @@ public final PrimitiveType primitiveType(final TypeKind k) {
         public boolean sameType(final TypeMirror t, final TypeMirror s) {
           return javacModelTypes.isSameType(t, s);
         }
-        /*
         @Override
-        public final TypeElement typeElement(final CharSequence m, final CharSequence n) {
-          return elements.getTypeElement(elements.getModuleElement(m), n);
+        public boolean subtype(final TypeMirror t, final TypeMirror s) {
+          return javacModelTypes.isSubtype(t, s);
         }
-        */
         @Override
         public final TypeElement typeElement(final CharSequence n) {
           return elements.getTypeElement(n);
@@ -248,6 +246,10 @@ public final PrimitiveType primitiveType(final TypeKind k) {
         public boolean sameType(final TypeMirror t, final TypeMirror s) {
           return javacModelTypes.isSameType(t, s);
         }
+        @Override
+        public boolean subtype(final TypeMirror t, final TypeMirror s) {
+          return javacModelTypes.isSubtype(t, s);
+        }
         /*
         @Override
         public final TypeElement typeElement(final CharSequence m, final CharSequence n) {
diff --git a/lang/src/test/java/org/microbean/lang/visitor/TestTypeClosureVisitor.java b/lang/src/test/java/org/microbean/lang/visitor/TestTypeClosureVisitor.java
index f366a8b9..0dd8ad83 100644
--- a/lang/src/test/java/org/microbean/lang/visitor/TestTypeClosureVisitor.java
+++ b/lang/src/test/java/org/microbean/lang/visitor/TestTypeClosureVisitor.java
@@ -95,7 +95,7 @@ final void testArrayTypeClosure() {
     final TypeMirror t = Lang.arrayTypeOf(Lang.declaredType("java.lang.Integer"));
     assertSame(TypeKind.ARRAY, t.getKind());
     
-    final Type integerArrayType = (Type)unwrap(Lang.arrayTypeOf(Lang.declaredType("java.lang.Integer")));
+    final Type integerArrayType = (Type)unwrap(t);
     assertSame(TypeKind.ARRAY, integerArrayType.getKind());
     final List<? extends TypeMirror> closure = javacCodeTypes.closure(integerArrayType);
     // Perhaps surprisingly, the closure (as implemented by javac) of any type, including array types, that is not a