diff --git a/.gitignore b/.gitignore
index b489d32..f9d5d37 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,5 @@
 /tests_results
 /main.exe
+/main.exe.mdb
 /*Generated.cs
 /*Generator.exe
\ No newline at end of file
diff --git a/LexerGenerator.cs b/LexerGenerator.cs
index bb51d5f..3aeba58 100644
--- a/LexerGenerator.cs
+++ b/LexerGenerator.cs
@@ -18,10 +18,10 @@ public static class LexerGenerator {
           Case("StringOpen"),
           Case("StringClose")),
         Record("Rule",
-          Field("S", "oldState"),
+          Field("Lexer.S", "oldState"),
           Field("string", "description"),
           Field("Func<GraphemeCluster, bool>", "test"),
-          Field("S", "throughState"),
-          Field("S", "newState"))));
+          Field("Lexer.S", "throughState"),
+          Field("Lexer.S", "newState"))));
   }
 }
\ No newline at end of file
diff --git a/Parser.cs b/Parser.cs
index 1e118eb..65b6d6c 100644
--- a/Parser.cs
+++ b/Parser.cs
@@ -8,32 +8,44 @@ using S = Lexer.S;
 using Lexeme = Lexer.Lexeme;
 using static Global;
 
-using PrecedenceDAG = System.Collections.Immutable.ImmutableDictionary<string, Parser.DAGNode>;
+using PrecedenceDAG = ImmutableDefaultDictionary<string, Parser.DAGNode>;
 
 public static partial class Parser {
-  public static PrecedenceDAG DefaultPrecedenceDAG = new PrecedenceDAG();
+  public static DAGNode EmptyDAGNode = new DAGNode(
+       infixLeftAssociative: ImmutableList<Operator>.Empty,
+       prefix:               ImmutableList<Operator>.Empty,
+       closed:               ImmutableList<Operator>.Empty,
+       terminal: ImmutableList<Operator>.Empty,
+       infixRightAssociative: ImmutableList<Operator>.Empty,
+       infixNonAssociative: ImmutableList<Operator>.Empty,
+       postfix: ImmutableList<Operator>.Empty,
+       successorNodes: ImmutableList<string>.Empty
+     );
 
-  public static DAGNode With(DAGNode node, Operator @operator) {
-    var newNode = @operator.fixity.Match(
-      Closed: () => node.lens.closed.Cons(@operator),
-      InfixLeftAssociative: () => node.lens.infixLeftAssociative.Cons(@operator),
-      InfixRightAssociative: () => node.lens.infixRightAssociative.Cons(@operator),
-      InfixNonAssociative: () => node.lens.infixNonAssociative.Cons(@operator),
-      Prefix: () => node.lens.prefix.Cons(@operator),
-      Postfix: () => node.lens.postfix.Cons(@operator),
-      Terminal: () => node.lens.terminal.Cons(@operator)
+  public static PrecedenceDAG DefaultPrecedenceDAG
+    = new PrecedenceDAG(EmptyDAGNode);
+
+  public static Whole With<Whole>(this ILens<DAGNode, Whole> node, Operator @operator) {
+    return @operator.fixity.Match(
+      Closed:
+        () => node.Closed().Cons(@operator),
+      InfixLeftAssociative:
+        () => node.InfixLeftAssociative().Cons(@operator),
+      InfixRightAssociative:
+        () => node.InfixRightAssociative().Cons(@operator),
+      InfixNonAssociative:
+        () => node.InfixNonAssociative().Cons(@operator),
+      Prefix:
+        () => node.Prefix().Cons(@operator),
+      Postfix:
+        () => node.Postfix().Cons(@operator),
+      Terminal:
+        () => node.Terminal().Cons(@operator)
     );
-    // op.fixity, parts, holes
-    throw new NotImplementedException();
   }
 
-  public static PrecedenceDAG With(PrecedenceDAG precedenceDAG, Operator @operator) {
-    precedenceDAG.lens(@operator.precedenceGroup);
-    /*precedenceDAG.update(
-      dagNode => dagNode.Add(@operator)
-    );*/
-    throw new NotImplementedException();
-  }
+  public static PrecedenceDAG With(PrecedenceDAG precedenceDAG, Operator @operator)
+    => precedenceDAG.lens()[@operator.precedenceGroup].With(@operator);
 
   public static void DagToGrammar(DAGNode precedenceDAG) {
     
diff --git a/T4/Generator.cs b/T4/Generator.cs
index 9e98141..a7fe0dd 100644
--- a/T4/Generator.cs
+++ b/T4/Generator.cs
@@ -19,6 +19,19 @@ public static class Generator {
 
       w("using System;");
       w($"{singleHeader}");
+      foreach (var type in types) {
+        var name = type.Key;
+        var kind = type.Value.Item1;
+        var components = type.Value.Item2;
+        switch (kind) {
+          case Kind.Record:
+            w.RecordUsing(header, footer, qualifier, name, @components);
+            break;
+          case Kind.Variant:
+            w.VariantUsing(header, footer, qualifier, name, @components);
+            break;
+        }
+      }
       w("");
       foreach (var type in types) {
         var name = type.Key;
diff --git a/T4/RecordGenerator.cs b/T4/RecordGenerator.cs
index 9aaec48..c0ecdcd 100644
--- a/T4/RecordGenerator.cs
+++ b/T4/RecordGenerator.cs
@@ -6,6 +6,15 @@ using System.Linq;
 using Record = System.Collections.Immutable.ImmutableDictionary<string, string>;
 
 public static class RecordGenerator {
+  private static void NewExampleComment(this Action<string> w, string qualifier, string name, Record record) {
+    w($"  /* To create an instance of {name}, write:");
+    w($"     new {name}(");
+    w(String.Join(",\n", record.Select(@field => 
+                $"       {@field.Key}: new {@field.Value}(…)")));
+    w($"     )");
+    w($"  */");
+  }
+
   private static void Fields(this Action<string> w, string qualifier, string name, Record record) {
     foreach (var @field in record) {
       var F = @field.Key;
@@ -69,7 +78,9 @@ public static class RecordGenerator {
   private static void Lenses(this Action<string> w, string qualifier, string name, Record record) {
     w($"    public sealed class Lens<Whole> : ILens<{name}, Whole> {{");
     w($"      public readonly System.Func<{name}, Whole> wrap;");
-    w($"      public readonly {name} oldHole;");
+    w($"      private readonly {name} oldHole;");
+    w($"");
+    w($"      public {name} value {{ get => oldHole; }}");
     w($"");
     w($"      public Lens(System.Func<{name}, Whole> wrap, {name} oldHole) {{");
     w($"        this.wrap = wrap;");
@@ -84,7 +95,8 @@ public static class RecordGenerator {
       w($"        => oldHole.{F}.ChainLens(");
       w($"          value => wrap(oldHole.With{caseF}(value)));");
     }
-    w($"      public Whole Update(Func<{name}, {name}> update) => wrap(update(oldHole));");
+    w($"      public Whole Update(Func<{name}, {name}> update)");
+    w($"        => wrap(update(oldHole));");
     w($"    }}");
   }
 
@@ -106,10 +118,42 @@ public static class RecordGenerator {
     w($"  }}");
   }
 
+  private static void LensExtensionMethods(this Action<string> w, string qualifier, string name, Record record) {
+    w($"    public static class {name}LensExtensionMethods {{");
+    foreach (var @field in record) {
+      var F = @field.Key;
+      var noAtF = F.StartsWith("@") ? F.Substring(1) : F;
+      var caseF = Char.ToUpper(noAtF[0]) + noAtF.Substring(1);
+      var Ty = @field.Value;
+      // same as {name}.Lens but as extension mehtods (should
+      // be extension properties once C# supports those) to
+      // be applied to instances of ILens<{name}, Whole>
+      w($"      public static ILens<{Ty}, Whole>");
+      w($"        {caseF}<Whole>(");
+      w($"          this ILens<{qualifier}{name}, Whole> self)");
+      w($"        => self.value.{F}.ChainLens(");
+      w($"          value => self.Update(oldHole => oldHole.With{caseF}(value)));");
+    }
+    w($"    }}");
+  }
+
   public static void Record(this Action<string> w, string header, string footer, string qualifier, string name, Record record) {
     w($"{header}");
     w("");
+    w.NewExampleComment(qualifier, name, record);
+    w("");
     w.RecordClass(qualifier, name, record);
     w($"{footer}");
+    w.LensExtensionMethods(qualifier, name, record);
+  }
+
+  private static void QualifierAliases(this Action<string> w, string qualifier, string name, Record record) {
+    if (qualifier != "") {
+      w($"using {name} = {qualifier}{name};");
+    }
+  }
+
+  public static void RecordUsing(this Action<string> w, string header, string footer, string qualifier, string name, Record record) {
+    w.QualifierAliases(qualifier, name, record);
   }
 }
diff --git a/T4/VariantGenerator.cs b/T4/VariantGenerator.cs
index 919290d..8111ebb 100644
--- a/T4/VariantGenerator.cs
+++ b/T4/VariantGenerator.cs
@@ -185,8 +185,20 @@ public static class VariantGenerator {
     w($"{header}");
     w($"");
     w.MatchExampleComment(qualifier, name, variant);
+    w($"");
     w.VariantClass(qualifier, name, variant);
+    w($"");
     w.ExtensionMethods(qualifier, name, variant);
     w($"{footer}");
   }
+
+  private static void QualifierAliases(this Action<string> w, string qualifier, string name, Variant variant) {
+    if (qualifier != "") {
+      w($"using {name} = {qualifier}{name};");
+    }
+  }
+
+  public static void VariantUsing(this Action<string> w, string header, string footer, string qualifier, string name, Variant variant) {
+    w.QualifierAliases(qualifier, name, variant);
+  }
 }
\ No newline at end of file
diff --git a/Utils/Immutable/DefaultDictionary.cs b/Utils/Immutable/DefaultDictionary.cs
index 2d76399..737f119 100644
--- a/Utils/Immutable/DefaultDictionary.cs
+++ b/Utils/Immutable/DefaultDictionary.cs
@@ -7,6 +7,11 @@ public class ImmutableDefaultDictionary<TKey, TValue> : IEnumerable<KeyValuePair
   public readonly TValue defaultValue;
   public readonly ImmutableDictionary<TKey, TValue> dictionary;
   
+  public ImmutableDefaultDictionary(TValue defaultValue) {
+    this.defaultValue = defaultValue;
+    this.dictionary = ImmutableDictionary<TKey, TValue>.Empty;
+  }
+
   public ImmutableDefaultDictionary(TValue defaultValue, ImmutableDictionary<TKey, TValue> dictionary) {
     this.defaultValue = defaultValue;
     this.dictionary = dictionary;
@@ -16,9 +21,15 @@ public class ImmutableDefaultDictionary<TKey, TValue> : IEnumerable<KeyValuePair
     get => dictionary.GetOrDefault(key, defaultValue);
   }
 
-  public ImmutableDefaultDictionary<TKey, TValue> With(TKey key, TValue value)
+  public ImmutableDefaultDictionary<TKey, TValue> Add(TKey key, TValue value)
     => new ImmutableDefaultDictionary<TKey, TValue>(defaultValue, dictionary.Add(key, value));
 
+  public ImmutableDefaultDictionary<TKey, TValue> SetItem(TKey key, TValue value)
+    => new ImmutableDefaultDictionary<TKey, TValue>(defaultValue, dictionary.SetItem(key, value));
+
+  public ImmutableDefaultDictionary<TKey, TValue> Remove(TKey key)
+    => new ImmutableDefaultDictionary<TKey, TValue>(defaultValue, dictionary.Remove(key));
+
   public IEnumerator<KeyValuePair<TKey, TValue>> GetEnumerator() => dictionary.GetEnumerator();
 
   IEnumerator IEnumerable.GetEnumerator() => dictionary.GetEnumerator();
diff --git a/Utils/Lens.cs b/Utils/Lens.cs
index 1780f1d..fb759f7 100644
--- a/Utils/Lens.cs
+++ b/Utils/Lens.cs
@@ -1,9 +1,8 @@
-// Code quality of this file: low.
-
 using System;
 using System.Collections.Immutable;
 
 public interface ILens<Hole, Whole> {
+  Hole value { get; }
   Whole Update(Func<Hole, Hole> update);
 }
 
@@ -14,12 +13,6 @@ public static class LensExtensionMethods {
   public static Whole Cons<T, Whole>(this ILens<ImmutableList<T>, Whole> lens, T value)
     => lens.Update(oldHole => oldHole.Cons(value));
 
-  public static ImmutableListLens<T, Whole>
-    ChainLens<T, Whole>(
-      this ImmutableList<T> hole,
-      System.Func<ImmutableList<T>, Whole> wrap)
-    => new ImmutableListLens<T, Whole>(wrap: wrap, oldHole: hole);
-
   public static ILens<string, Whole> ChainLens<Whole>(this string hole, System.Func<string, Whole> wrap) => new LeafLens<string, Whole>(wrap: wrap, oldHole: hole);
 
   public static ILens<Func<GraphemeCluster,bool>, Whole> ChainLens<Whole>(this Func<GraphemeCluster,bool> hole, System.Func<Func<GraphemeCluster,bool>, Whole> wrap) => new LeafLens<Func<GraphemeCluster,bool>, Whole>(wrap: wrap, oldHole: hole);
diff --git a/Utils/Lens/ImmutableDefaultDictionaryLens.cs b/Utils/Lens/ImmutableDefaultDictionaryLens.cs
new file mode 100644
index 0000000..93cf184
--- /dev/null
+++ b/Utils/Lens/ImmutableDefaultDictionaryLens.cs
@@ -0,0 +1,86 @@
+// Code quality of this file: medium.
+
+using System;
+using System.Collections.Immutable;
+
+public sealed class ImmutableDefaultDictionaryValueLens<TKey, TValue, Whole> : ILens<TValue, Whole> {
+  public readonly Func<ImmutableDefaultDictionary<TKey, TValue>, Whole> wrap;
+  
+  private readonly ImmutableDefaultDictionary<TKey, TValue> oldDictionary;
+  
+  private readonly TKey oldKey;
+
+  public TValue value { get => oldDictionary[oldKey]; }
+
+  public ImmutableDefaultDictionaryValueLens(Func<ImmutableDefaultDictionary<TKey, TValue>, Whole> wrap, ImmutableDefaultDictionary<TKey, TValue> oldDictionary, TKey oldKey) {
+    // TODO: check that key exists.
+    this.wrap = wrap;
+    this.oldDictionary = oldDictionary;
+    this.oldKey = oldKey;
+  }
+
+  // Put methods with the following signature here to focus on sub-parts of the list as needed.
+  // public ILens<ImmutableDefaultDictionary<T>,Whole> sub-part => oldHole.sub-part.ChainLens(value => oldHole.with-sub-part(value));
+
+  public Whole Update(Func<TValue, TValue> update) {
+    var oldValue = oldDictionary[oldKey];
+    return wrap(oldDictionary.SetItem(oldKey, update(oldValue)));
+  }
+
+  public ImmutableDefaultDictionaryValueLens<TKey, TValue, Whole> UpdateKey(Func<TKey, TKey> update) {
+    var newKey = update(oldKey);
+    return new ImmutableDefaultDictionaryValueLens<TKey, TValue, Whole>(
+      wrap,
+      oldDictionary.Remove(oldKey).Add(newKey, oldDictionary[oldKey]),
+      newKey);
+  }
+}
+
+public sealed class ImmutableDefaultDictionaryLens<TKey, TValue, Whole> : ILens<ImmutableDefaultDictionary<TKey, TValue>, Whole> {
+  public readonly Func<ImmutableDefaultDictionary<TKey, TValue>, Whole> wrap;
+  private readonly ImmutableDefaultDictionary<TKey, TValue> oldHole;
+
+  public ImmutableDefaultDictionary<TKey, TValue> value { get => oldHole; }
+
+  public ImmutableDefaultDictionaryLens(Func<ImmutableDefaultDictionary<TKey, TValue>, Whole> wrap, ImmutableDefaultDictionary<TKey, TValue> oldHole) {
+    // TODO: check that key exists.
+    this.wrap = wrap;
+    this.oldHole = oldHole;
+  }
+
+  // Put methods with the following signature here to focus on sub-parts of the list as needed.
+  public ImmutableDefaultDictionaryValueLens<TKey, TValue, Whole> this[TKey key] {
+    get => new ImmutableDefaultDictionaryValueLens<TKey, TValue, Whole>(wrap, oldHole, key);
+  }
+  
+  public Whole Update(Func<ImmutableDefaultDictionary<TKey, TValue>, ImmutableDefaultDictionary<TKey, TValue>> update) {
+    return wrap(update(oldHole));
+  }
+}
+
+public static class ImmutableDefaultDictionaryLensExtensionMethods {
+  public static ImmutableDefaultDictionaryLens<TKey, TValue, Whole>
+    ChainLens<TKey, TValue, Whole>(
+      this ImmutableDefaultDictionary<TKey, TValue> hole,
+      System.Func<ImmutableDefaultDictionary<TKey, TValue>, Whole> wrap)
+    => new ImmutableDefaultDictionaryLens<TKey, TValue, Whole>(wrap: wrap, oldHole: hole);
+
+  // TODO: this should be an extension property (once C# supports them)
+  public static ImmutableDefaultDictionaryLens<TKey, TValue, ImmutableDefaultDictionary<TKey, TValue>>
+    lens<TKey, TValue>(
+      this ImmutableDefaultDictionary<TKey, TValue> d)
+    => d.ChainLens(x => x);
+
+  // this is a shorthand since we don't have extension properties
+  public static ImmutableDefaultDictionaryValueLens<TKey, TValue, ImmutableDefaultDictionary<TKey, TValue>>
+    lens<TKey, TValue>(
+      this ImmutableDefaultDictionary<TKey, TValue> d,
+      TKey key)
+    => d.lens()[key];
+
+  public static ImmutableDefaultDictionaryValueLens<TKey, TValue, Whole>
+    UpdateKey<TKey, TValue, Whole>(
+      this ImmutableDefaultDictionaryValueLens<TKey, TValue, Whole> lens,
+      TKey newKey)
+    => lens.UpdateKey(oldKey => newKey);
+}
\ No newline at end of file
diff --git a/Utils/Lens/ImmutableDictionaryLens.cs b/Utils/Lens/ImmutableDictionaryLens.cs
deleted file mode 100644
index 4a48b90..0000000
--- a/Utils/Lens/ImmutableDictionaryLens.cs
+++ /dev/null
@@ -1,63 +0,0 @@
-using System;
-using System.Collections.Immutable;
-
-public sealed class ImmutableDictionaryValueLens<TKey, TValue, Whole> : ILens<TValue, Whole> {
-  public readonly Func<ImmutableDictionary<TKey, TValue>, Whole> wrap;
-  public readonly ImmutableDictionary<TKey, TValue> oldDictionary;
-  public readonly TKey oldKey;
-
-  public ImmutableDictionaryValueLens(Func<ImmutableDictionary<TKey, TValue>, Whole> wrap, ImmutableDictionary<TKey, TValue> oldDictionary, TKey oldKey) {
-    // TODO: check that key exists.
-    this.wrap = wrap;
-    this.oldDictionary = oldDictionary;
-    this.oldKey = oldKey;
-  }
-
-  // Put methods with the following signature here to focus on sub-parts of the list as needed.
-  // public ILens<ImmutableList<T>,Whole> sub-part => oldHole.sub-part.ChainLens(value => oldHole.with-sub-part(value));
-
-  public Whole Update(Func<TValue, TValue> update) {
-    var oldValue = oldDictionary[oldKey];
-    return wrap(oldDictionary.SetItem(oldKey, update(oldValue)));
-  }
-
-  public ImmutableDictionaryValueLens<TKey, TValue, Whole> UpdateKey(Func<TKey, TKey> update) {
-    var newKey = update(oldKey);
-    return new ImmutableDictionaryValueLens<TKey, TValue, Whole>(
-      wrap,
-      oldDictionary.Remove(oldKey).Add(newKey, oldDictionary[oldKey]),
-      newKey);
-  }
-}
-
-public sealed class ImmutableDictionaryLens<TKey, TValue, Whole> : ILens<ImmutableDictionary<TKey, TValue>, Whole> {
-  public readonly Func<ImmutableDictionary<TKey, TValue>, Whole> wrap;
-  public readonly ImmutableDictionary<TKey, TValue> oldHole;
-
-  public ImmutableDictionaryLens(Func<ImmutableDictionary<TKey, TValue>, Whole> wrap, ImmutableDictionary<TKey, TValue> oldHole) {
-    // TODO: check that key exists.
-    this.wrap = wrap;
-    this.oldHole = oldHole;
-  }
-
-  // Put methods with the following signature here to focus on sub-parts of the list as needed.
-  public ImmutableDictionaryValueLens<TKey, TValue, Whole> this[TKey key] {
-    get => new ImmutableDictionaryValueLens<TKey, TValue, Whole>(wrap, oldHole, key);
-  }
-  
-  public Whole Update(Func<ImmutableDictionary<TKey, TValue>, ImmutableDictionary<TKey, TValue>> update) {
-    return wrap(update(oldHole));
-  }
-}
-
-public static class ImmutableDictionaryLensExtensionMethods {
-  public static ImmutableDictionaryLens<TKey, TValue, ImmutableDictionary<TKey, TValue>> lens<TKey, TValue>(this ImmutableDictionary<TKey, TValue> d)
-    => new ImmutableDictionaryLens<TKey, TValue, ImmutableDictionary<TKey, TValue>>(x => x, d);
-
-  public static ImmutableDictionaryValueLens<TKey, TValue, Whole> UpdateKey<TKey, TValue, Whole, Whole>(this ImmutableDictionaryValueLens<TKey, TValue, Whole> lens, TKey newKey)
-    => lens.UpdateKey(oldKey => newKey);
-
-  // This would need an IFocusable<TValue> constraint which is hard to get
-  //public static ILens<ImmutableDictionary<TKey, ?>, Whole> ChainLens<TKey, Whole>(this ImmutableDictionary<TKey, ?> hole, System.Func<ImmutableDictionary<TKey, ?>, Whole> wrap)
-  //  => new ImmutableDictionaryLens<TKey, ?, Whole>(wrap: wrap, oldHole: hole);
-}
\ No newline at end of file
diff --git a/Utils/Lens/ImmutableListLens.cs b/Utils/Lens/ImmutableListLens.cs
index 118e2cb..30cd4d1 100644
--- a/Utils/Lens/ImmutableListLens.cs
+++ b/Utils/Lens/ImmutableListLens.cs
@@ -3,7 +3,9 @@ using System.Collections.Immutable;
 
 public sealed class ImmutableListLens<T, Whole> : ILens<ImmutableList<T>, Whole> {
   public readonly Func<ImmutableList<T>, Whole> wrap;
-  public readonly ImmutableList<T> oldHole;
+  private readonly ImmutableList<T> oldHole;
+
+  public ImmutableList<T> value { get => oldHole; }
 
   public ImmutableListLens(Func<ImmutableList<T>, Whole> wrap, ImmutableList<T> oldHole) {
     this.wrap = wrap;
@@ -11,12 +13,21 @@ public sealed class ImmutableListLens<T, Whole> : ILens<ImmutableList<T>, Whole>
   }
 
   // Put methods with the following signature here to focus on sub-parts of the list as needed.
-  // public ILens<ImmutableList<T>,Whole> sub-part => oldHole.sub-part.ChainLens(value => oldHole.with-sub-part(value));
+  // public ILens<ImmutableList<T>,Whole> sub-part
+  //   => oldHole.sub-part.ChainLens(value => oldHole.with-sub-part(value));
 
   public Whole Update(Func<ImmutableList<T>, ImmutableList<T>> update) => wrap(update(oldHole));
 }
 
 public static class ImmutableListLensExtensionMethods {
-  public static ILens<ImmutableList<string>, Whole> ChainLens<Whole>(this ImmutableList<string> hole, System.Func<ImmutableList<string>, Whole> wrap)
-    => new ImmutableListLens<string, Whole>(wrap: wrap, oldHole: hole);
-}
\ No newline at end of file
+  public static ImmutableListLens<T, Whole>
+    ChainLens<T, Whole>(
+      this ImmutableList<T> hole,
+      System.Func<ImmutableList<T>, Whole> wrap)
+    => new ImmutableListLens<T, Whole>(wrap: wrap, oldHole: hole);
+
+  public static ImmutableListLens<T, ImmutableList<T>>
+    lens<T>(
+      this ImmutableList<T> d)
+    => d.ChainLens(x => x);
+}
diff --git a/Utils/Lens/LeafLens.cs b/Utils/Lens/LeafLens.cs
index d079acc..10ab6cf 100644
--- a/Utils/Lens/LeafLens.cs
+++ b/Utils/Lens/LeafLens.cs
@@ -5,7 +5,9 @@ using System.Collections.Immutable;
 // interesting to further focus.
 public sealed class LeafLens<T, Whole> : ILens<T, Whole> {
   public readonly System.Func<T, Whole> wrap;
-  public readonly T oldHole;
+  private readonly T oldHole;
+
+  public T value { get => oldHole; }
 
   public LeafLens(System.Func<T, Whole> wrap, T oldHole) {
     this.wrap = wrap;