diff --git a/Sources/SynKit/Libraries/Parser/ParseError.cs b/Sources/SynKit/Libraries/Parser/ParseError.cs
index 78f6e0c0..5dcd99e0 100644
--- a/Sources/SynKit/Libraries/Parser/ParseError.cs
+++ b/Sources/SynKit/Libraries/Parser/ParseError.cs
@@ -4,8 +4,6 @@
 
 using System;
 using System.Collections.Generic;
-using System.Collections.Generic.Polyfill;
-using System.Linq;
 
 namespace Yoakke.SynKit.Parser;
 
@@ -14,10 +12,12 @@ namespace Yoakke.SynKit.Parser;
 /// </summary>
 public class ParseError
 {
+    private ParseErrorElementDictionary elements;
+
     /// <summary>
     /// The error cases in different parse contexts.
     /// </summary>
-    public IReadOnlyDictionary<string, ParseErrorElement> Elements { get; }
+    public IReadOnlyDictionary<string, ParseErrorElement> Elements => this.elements;
 
     /// <summary>
     /// The item that was found, if any.
@@ -37,13 +37,13 @@ public class ParseError
     /// <param name="position">The position where the error occurred.</param>
     /// <param name="context">The context in which the error occurred.</param>
     public ParseError(object expected, object? got, IComparable position, string context)
-        : this(new Dictionary<string, ParseErrorElement> { { context, new ParseErrorElement(expected, context) } }, got, position)
+        : this(new ParseErrorElementDictionary(context, new ParseErrorElement(expected, context)), got, position)
     {
     }
 
-    private ParseError(IReadOnlyDictionary<string, ParseErrorElement> elements, object? got, IComparable position)
+    private ParseError(ParseErrorElementDictionary elements, object? got, IComparable position)
     {
-        this.Elements = elements;
+        this.elements = elements;
         this.Got = got;
         this.Position = position;
     }
@@ -65,19 +65,7 @@ private ParseError(IReadOnlyDictionary<string, ParseErrorElement> elements, obje
         if (cmp < 0) return second;
         if (cmp > 0) return first;
         // Both of them got stuck at the same place, merge entries
-        var elements = first.Elements.Values.ToDictionary(e => e.Context, e => new ParseErrorElement(e.Expected.ToHashSet(), e.Context));
-        foreach (var element in second.Elements.Values)
-        {
-            if (elements.TryGetValue(element.Context, out var part))
-            {
-                foreach (var e in element.Expected) part.Expected.Add(e);
-            }
-            else
-            {
-                part = new (element.Expected.ToHashSet(), element.Context);
-                elements.Add(element.Context, part);
-            }
-        }
+        var elements = first.elements.Merge(second.elements);
         // TODO: Think this through
         // NOTE: Could it ever happen that first.Got and second.Got are different but neither are null?
         // Would we want to unify these and move them to ParseErrorElement or something?
diff --git a/Sources/SynKit/Libraries/Parser/ParseErrorElementDictionary.cs b/Sources/SynKit/Libraries/Parser/ParseErrorElementDictionary.cs
new file mode 100644
index 00000000..c908badb
--- /dev/null
+++ b/Sources/SynKit/Libraries/Parser/ParseErrorElementDictionary.cs
@@ -0,0 +1,94 @@
+// Copyright (c) 2021-2022 Yoakke.
+// Licensed under the Apache License, Version 2.0.
+// Source repository: https://github.com/LanguageDev/Yoakke
+
+using System;
+using System.Collections;
+using System.Collections.Generic;
+using System.Collections.Generic.Polyfill;
+using System.Linq;
+
+namespace Yoakke.SynKit.Parser;
+
+internal class ParseErrorElementDictionary : IReadOnlyDictionary<string, ParseErrorElement>
+{
+    private string? firstKey;
+    private ParseErrorElement? firstItem;
+    private Dictionary<string, ParseErrorElement>? elements;
+
+    private ParseErrorElementDictionary()
+    {
+    }
+
+    private ParseErrorElementDictionary(Dictionary<string, ParseErrorElement> elements)
+    {
+        this.elements = elements;
+    }
+
+    public ParseErrorElementDictionary(string key, ParseErrorElement value)
+    {
+        this.firstKey = key;
+        this.firstItem = value;
+    }
+
+    public ParseErrorElement this[string key] => this.firstKey is null
+        ? this.elements is null
+            ? throw new KeyNotFoundException($"The key {key} was not found in the dictionary")
+            : this.elements[key]
+        : this.firstItem!;
+
+    public IEnumerable<string> Keys => this.firstKey is null ? this.elements!.Keys : new[] { this.firstKey };
+
+    public IEnumerable<ParseErrorElement> Values => this.firstKey is null ? this.elements!.Values : new[] { this.firstItem! };
+
+    public int Count => this.firstKey is null ? this.elements is null ? 0 : this.elements.Count : 1;
+
+    public bool ContainsKey(string key) => this.firstKey is null ? this.elements is null ? false : this.elements.ContainsKey(key) : this.firstKey == key;
+    public IEnumerator<KeyValuePair<string, ParseErrorElement>> GetEnumerator() => throw new NotImplementedException();
+    public bool TryGetValue(string key, out ParseErrorElement value) => throw new NotImplementedException();
+    IEnumerator IEnumerable.GetEnumerator() => this.GetEnumerator();
+
+    public ParseErrorElementDictionary Merge(ParseErrorElementDictionary other)
+    {
+        if (this.elements is null)
+        {
+            if (other.elements is null)
+            {
+                if (this.firstKey == other.firstKey)
+                {
+                    var newExpected = other.firstItem!.Expected.ToHashSet();
+                    newExpected.UnionWith(this.firstItem!.Expected);
+                    return new(other.firstKey!, new (newExpected, this.firstItem.Context));
+                }
+                else
+                {
+                    return new(new Dictionary<string, ParseErrorElement>
+                    {
+                        { this.firstKey!, this.firstItem! },
+                        { other.firstKey!, other.firstItem! },
+                    });
+                }
+            }
+            else
+            {
+                return new(new (other.elements));
+            }
+        }
+
+        var elements = this.elements.Values.ToDictionary(e => e.Context, e => new ParseErrorElement(e.Expected.ToHashSet(), e.Context));
+        foreach (var element in other.Values)
+        {
+            if (elements.TryGetValue(element.Context, out var part))
+            {
+                foreach (var e in element.Expected) part.Expected.Add(e);
+            }
+            else
+            {
+                part = new(element.Expected.ToHashSet(), element.Context);
+                elements.Add(element.Context, part);
+            }
+        }
+
+        return new (elements);
+    }
+}