diff --git a/ICSharpCode.NRefactory/ICSharpCode.NRefactory.csproj b/ICSharpCode.NRefactory/ICSharpCode.NRefactory.csproj
index 5d34c8a676..610613fede 100644
--- a/ICSharpCode.NRefactory/ICSharpCode.NRefactory.csproj
+++ b/ICSharpCode.NRefactory/ICSharpCode.NRefactory.csproj
@@ -151,12 +151,14 @@
     <Compile Include="TypeSystem\Implementation\BitVector16.cs" />
     <Compile Include="TypeSystem\Implementation\DefaultExplicitInterfaceImplementation.cs" />
     <Compile Include="TypeSystem\Implementation\DefaultParameter.cs" />
+    <Compile Include="TypeSystem\Implementation\DynamicType.cs" />
     <Compile Include="TypeSystem\Implementation\GetClassTypeReference.cs" />
     <Compile Include="TypeSystem\Implementation\NestedTypeReference.cs" />
     <Compile Include="TypeSystem\Implementation\PointerType.cs" />
     <Compile Include="TypeSystem\Implementation\ProxyTypeResolveContext.cs" />
     <Compile Include="TypeSystem\Implementation\DefaultTypeDefinition.cs" />
     <Compile Include="TypeSystem\Implementation\NullType.cs" />
+    <Compile Include="TypeSystem\Implementation\SimpleConstantValue.cs" />
     <Compile Include="TypeSystem\Implementation\TypeWithElementType.cs" />
     <Compile Include="TypeSystem\Implementation\UnknownType.cs" />
     <Compile Include="TypeSystem\INamedElement.cs" />
diff --git a/ICSharpCode.NRefactory/TypeSystem/CecilProjectContent.cs b/ICSharpCode.NRefactory/TypeSystem/CecilProjectContent.cs
index fe7e6fb54f..384a4f0d80 100644
--- a/ICSharpCode.NRefactory/TypeSystem/CecilProjectContent.cs
+++ b/ICSharpCode.NRefactory/TypeSystem/CecilProjectContent.cs
@@ -166,8 +166,7 @@ namespace ICSharpCode.NRefactory.TypeSystem
 				return new ConstructedReturnType(baseType, para);*/
 				throw new NotImplementedException();
 			} else if (type is GenericParameter) {
-				throw new NotImplementedException();
-				/*GenericParameter typeGP = type as GenericParameter;
+				GenericParameter typeGP = type as GenericParameter;
 				if (typeGP.Owner is MethodDefinition) {
 					IMethod method = entity as IMethod;
 					if (method != null) {
@@ -184,7 +183,7 @@ namespace ICSharpCode.NRefactory.TypeSystem
 						}
 					}
 					return SharedTypes.UnknownType;
-				}*/
+				}
 			} else {
 				string name = type.FullName;
 				if (name == null)
@@ -331,33 +330,12 @@ namespace ICSharpCode.NRefactory.TypeSystem
 		#region Read Constant Value
 		static IConstantValue ReadConstantValue(CustomAttributeArgument arg, ITypeResolveContext earlyBindContext)
 		{
-			return new CecilConstantValue(arg.Type, arg.Value, earlyBindContext);
-		}
-		
-		sealed class CecilConstantValue : Immutable, IConstantValue
-		{
-			readonly ITypeReference type;
-			readonly object value;
-			
-			public CecilConstantValue(TypeReference type, object value, ITypeResolveContext earlyBindContext)
-			{
-				this.type = ReadTypeReference(type, earlyBindContext: earlyBindContext);
-				TypeReference valueType = value as TypeReference;
-				if (valueType != null)
-					this.value = ReadTypeReference(valueType, earlyBindContext: earlyBindContext);
-				else
-					this.value = value;
-			}
-			
-			public IType GetValueType(ITypeResolveContext context)
-			{
-				return type.Resolve(context);
-			}
-			
-			public object GetValue(ITypeResolveContext context)
-			{
-				return value;
-			}
+			ITypeReference type = ReadTypeReference(arg.Type, earlyBindContext: earlyBindContext);
+			object value = arg.Value;
+			TypeReference valueType = value as TypeReference;
+			if (valueType != null)
+				value = ReadTypeReference(valueType, earlyBindContext: earlyBindContext);
+			return new SimpleConstantValue(type, value);
 		}
 		#endregion
 		
diff --git a/ICSharpCode.NRefactory/TypeSystem/ITypeParameter.cs b/ICSharpCode.NRefactory/TypeSystem/ITypeParameter.cs
index 6136095bfa..4cd2d62252 100644
--- a/ICSharpCode.NRefactory/TypeSystem/ITypeParameter.cs
+++ b/ICSharpCode.NRefactory/TypeSystem/ITypeParameter.cs
@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics.Contracts;
+using ICSharpCode.NRefactory.TypeSystem;
 
 namespace ICSharpCode.NRefactory.TypeSystem
 {
@@ -11,13 +12,8 @@ namespace ICSharpCode.NRefactory.TypeSystem
 	/// Type parameter of a generic class/method.
 	/// </summary>
 	[ContractClass(typeof(ITypeParameterContract))]
-	public interface ITypeParameter : IFreezable
+	public interface ITypeParameter : IType, IFreezable
 	{
-		/// <summary>
-		/// The name of the type parameter (for example "T")
-		/// </summary>
-		string Name { get; }
-		
 		/// <summary>
 		/// Gets the index of the type parameter in the type parameter list of the owning method/class.
 		/// </summary>
@@ -30,14 +26,13 @@ namespace ICSharpCode.NRefactory.TypeSystem
 		
 		/// <summary>
 		/// The method this type parameter is defined for.
-		/// This property is null when the type parameter is for a class.
+		/// This property returns null if the type parameter belong to a class.
 		/// </summary>
 		IMethod ParentMethod { get; }
 		
 		/// <summary>
 		/// The class this type parameter is defined for.
-		/// When the type parameter is defined for a method, this is the class containing
-		/// that method.
+		/// This property returns null if the type parameter belong to a method.
 		/// </summary>
 		ITypeDefinition ParentClass { get; }
 		
@@ -61,6 +56,11 @@ namespace ICSharpCode.NRefactory.TypeSystem
 		/// </summary>
 		bool HasValueTypeConstraint { get; }
 		
+		/// <summary>
+		/// Gets the variance of this type parameter.
+		/// </summary>
+		VarianceModifier Variance { get; }
+		
 		/// <summary>
 		/// Gets the type that was used to bind this type parameter.
 		/// This property returns null for generic methods/classes, it
@@ -74,17 +74,28 @@ namespace ICSharpCode.NRefactory.TypeSystem
 		ITypeParameter UnboundTypeParameter { get; }
 	}
 	
+	/// <summary>
+	/// Represents the variance of a type parameter.
+	/// </summary>
+	public enum VarianceModifier
+	{
+		/// <summary>
+		/// The type parameter is not variant.
+		/// </summary>
+		Invariant,
+		/// <summary>
+		/// The type parameter is covariant (used in output position).
+		/// </summary>
+		Covariant,
+		/// <summary>
+		/// The type parameter is contravariant (used in input position).
+		/// </summary>
+		Contravariant
+	};
 	
 	[ContractClassFor(typeof(ITypeParameter))]
-	abstract class ITypeParameterContract : IFreezableContract, ITypeParameter
+	abstract class ITypeParameterContract : ITypeContract, ITypeParameter
 	{
-		string ITypeParameter.Name {
-			get {
-				Contract.Ensures(Contract.Result<string>() != null);
-				return null;
-			}
-		}
-		
 		int ITypeParameter.Index {
 			get {
 				Contract.Ensures(Contract.Result<int>() >= 0);
@@ -107,7 +118,6 @@ namespace ICSharpCode.NRefactory.TypeSystem
 		
 		ITypeDefinition ITypeParameter.ParentClass {
 			get {
-				Contract.Ensures(Contract.Result<ITypeDefinition>() != null);
 				return null;
 			}
 		}
@@ -142,5 +152,9 @@ namespace ICSharpCode.NRefactory.TypeSystem
 				return null;
 			}
 		}
+		
+		VarianceModifier ITypeParameter.Variance {
+			get { return VarianceModifier.Invariant; }
+		}
 	}
 }
diff --git a/ICSharpCode.NRefactory/TypeSystem/Implementation/AbstractFreezable.cs b/ICSharpCode.NRefactory/TypeSystem/Implementation/AbstractFreezable.cs
index 51839cf90c..23439da204 100644
--- a/ICSharpCode.NRefactory/TypeSystem/Implementation/AbstractFreezable.cs
+++ b/ICSharpCode.NRefactory/TypeSystem/Implementation/AbstractFreezable.cs
@@ -1,9 +1,5 @@
-// <file>
-//     <copyright see="prj:///doc/copyright.txt"/>
-//     <license see="prj:///doc/license.txt"/>
-//     <author name="Daniel Grunwald"/>
-//     <version>$Revision$</version>
-// </file>
+// Copyright (c) 2010 AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
+// This code is distributed under MIT X11 license (for details please see \doc\license.txt)
 
 using System;
 using System.Collections.Generic;
diff --git a/ICSharpCode.NRefactory/TypeSystem/Implementation/AbstractType.cs b/ICSharpCode.NRefactory/TypeSystem/Implementation/AbstractType.cs
index 2080495803..1aeb56fd4e 100644
--- a/ICSharpCode.NRefactory/TypeSystem/Implementation/AbstractType.cs
+++ b/ICSharpCode.NRefactory/TypeSystem/Implementation/AbstractType.cs
@@ -1,9 +1,5 @@
-// <file>
-//     <copyright see="prj:///doc/copyright.txt"/>
-//     <license see="prj:///doc/license.txt"/>
-//     <author name="Daniel Grunwald"/>
-//     <version>$Revision$</version>
-// </file>
+// Copyright (c) 2010 AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
+// This code is distributed under MIT X11 license (for details please see \doc\license.txt)
 
 using System;
 using System.Collections.Generic;
diff --git a/ICSharpCode.NRefactory/TypeSystem/Implementation/AbstractTypeReference.cs b/ICSharpCode.NRefactory/TypeSystem/Implementation/AbstractTypeReference.cs
index a3f71865a8..b395018121 100644
--- a/ICSharpCode.NRefactory/TypeSystem/Implementation/AbstractTypeReference.cs
+++ b/ICSharpCode.NRefactory/TypeSystem/Implementation/AbstractTypeReference.cs
@@ -1,9 +1,5 @@
-// <file>
-//     <copyright see="prj:///doc/copyright.txt"/>
-//     <license see="prj:///doc/license.txt"/>
-//     <author name="Daniel Grunwald"/>
-//     <version>$Revision$</version>
-// </file>
+// Copyright (c) 2010 AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
+// This code is distributed under MIT X11 license (for details please see \doc\license.txt)
 
 using System;
 using System.Collections.Generic;
diff --git a/ICSharpCode.NRefactory/TypeSystem/Implementation/BitVector16.cs b/ICSharpCode.NRefactory/TypeSystem/Implementation/BitVector16.cs
index ad4aeb45ca..80bbb7ab3e 100644
--- a/ICSharpCode.NRefactory/TypeSystem/Implementation/BitVector16.cs
+++ b/ICSharpCode.NRefactory/TypeSystem/Implementation/BitVector16.cs
@@ -1,9 +1,6 @@
-// <file>
-//     <copyright see="prj:///doc/copyright.txt"/>
-//     <license see="prj:///doc/license.txt"/>
-//     <author name="Daniel Grunwald"/>
-//     <version>$Revision$</version>
-// </file>
+// Copyright (c) 2010 AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
+// This code is distributed under MIT X11 license (for details please see \doc\license.txt)
+
 using System;
 
 namespace ICSharpCode.NRefactory.TypeSystem.Implementation
diff --git a/ICSharpCode.NRefactory/TypeSystem/Implementation/DefaultExplicitInterfaceImplementation.cs b/ICSharpCode.NRefactory/TypeSystem/Implementation/DefaultExplicitInterfaceImplementation.cs
index 51d4299990..61ae2e6504 100644
--- a/ICSharpCode.NRefactory/TypeSystem/Implementation/DefaultExplicitInterfaceImplementation.cs
+++ b/ICSharpCode.NRefactory/TypeSystem/Implementation/DefaultExplicitInterfaceImplementation.cs
@@ -1,9 +1,5 @@
-// <file>
-//     <copyright see="prj:///doc/copyright.txt"/>
-//     <license see="prj:///doc/license.txt"/>
-//     <author name="Daniel Grunwald"/>
-//     <version>$Revision$</version>
-// </file>
+// Copyright (c) 2010 AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
+// This code is distributed under MIT X11 license (for details please see \doc\license.txt)
 
 using System;
 
diff --git a/ICSharpCode.NRefactory/TypeSystem/Implementation/DefaultParameter.cs b/ICSharpCode.NRefactory/TypeSystem/Implementation/DefaultParameter.cs
index dfd498c1b7..532a00db9c 100644
--- a/ICSharpCode.NRefactory/TypeSystem/Implementation/DefaultParameter.cs
+++ b/ICSharpCode.NRefactory/TypeSystem/Implementation/DefaultParameter.cs
@@ -1,9 +1,5 @@
-// <file>
-//     <copyright see="prj:///doc/copyright.txt"/>
-//     <license see="prj:///doc/license.txt"/>
-//     <author name="Daniel Grunwald"/>
-//     <version>$Revision$</version>
-// </file>
+// Copyright (c) 2010 AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
+// This code is distributed under MIT X11 license (for details please see \doc\license.txt)
 
 using System;
 using System.Collections.Generic;
diff --git a/ICSharpCode.NRefactory/TypeSystem/Implementation/DefaultTypeDefinition.cs b/ICSharpCode.NRefactory/TypeSystem/Implementation/DefaultTypeDefinition.cs
index ed6f5c7968..4abf28e3d6 100644
--- a/ICSharpCode.NRefactory/TypeSystem/Implementation/DefaultTypeDefinition.cs
+++ b/ICSharpCode.NRefactory/TypeSystem/Implementation/DefaultTypeDefinition.cs
@@ -1,9 +1,5 @@
-// <file>
-//     <copyright see="prj:///doc/copyright.txt"/>
-//     <license see="prj:///doc/license.txt"/>
-//     <author name="Daniel Grunwald"/>
-//     <version>$Revision$</version>
-// </file>
+// Copyright (c) 2010 AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
+// This code is distributed under MIT X11 license (for details please see \doc\license.txt)
 
 using System;
 using System.Collections.Generic;
diff --git a/ICSharpCode.NRefactory/TypeSystem/Implementation/DynamicType.cs b/ICSharpCode.NRefactory/TypeSystem/Implementation/DynamicType.cs
new file mode 100644
index 0000000000..fe9cceb509
--- /dev/null
+++ b/ICSharpCode.NRefactory/TypeSystem/Implementation/DynamicType.cs
@@ -0,0 +1,31 @@
+// Copyright (c) 2010 AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
+// This code is distributed under MIT X11 license (for details please see \doc\license.txt)
+
+using System;
+
+namespace ICSharpCode.NRefactory.TypeSystem.Implementation
+{
+	/// <summary>
+	/// Type representing the C# 'dynamic' type.
+	/// </summary>
+	sealed class DynamicType : AbstractType
+	{
+		public override string Name {
+			get { return "dynamic"; }
+		}
+		
+		public override bool? IsReferenceType {
+			get { return true; }
+		}
+		
+		public override bool Equals(IType other)
+		{
+			return other is DynamicType;
+		}
+		
+		public override int GetHashCode()
+		{
+			return 31986112;
+		}
+	}
+}
diff --git a/ICSharpCode.NRefactory/TypeSystem/Implementation/GetClassTypeReference.cs b/ICSharpCode.NRefactory/TypeSystem/Implementation/GetClassTypeReference.cs
index 9314fcf4a9..372a327da2 100644
--- a/ICSharpCode.NRefactory/TypeSystem/Implementation/GetClassTypeReference.cs
+++ b/ICSharpCode.NRefactory/TypeSystem/Implementation/GetClassTypeReference.cs
@@ -1,9 +1,6 @@
-// <file>
-//     <copyright see="prj:///doc/copyright.txt"/>
-//     <license see="prj:///doc/license.txt"/>
-//     <author name="Daniel Grunwald"/>
-//     <version>$Revision$</version>
-// </file>
+// Copyright (c) 2010 AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
+// This code is distributed under MIT X11 license (for details please see \doc\license.txt)
+
 using System;
 
 namespace ICSharpCode.NRefactory.TypeSystem.Implementation
diff --git a/ICSharpCode.NRefactory/TypeSystem/Implementation/NestedTypeReference.cs b/ICSharpCode.NRefactory/TypeSystem/Implementation/NestedTypeReference.cs
index f42e25b057..420e753aa3 100644
--- a/ICSharpCode.NRefactory/TypeSystem/Implementation/NestedTypeReference.cs
+++ b/ICSharpCode.NRefactory/TypeSystem/Implementation/NestedTypeReference.cs
@@ -1,9 +1,6 @@
-// <file>
-//     <copyright see="prj:///doc/copyright.txt"/>
-//     <license see="prj:///doc/license.txt"/>
-//     <author name="Daniel Grunwald"/>
-//     <version>$Revision$</version>
-// </file>
+// Copyright (c) 2010 AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
+// This code is distributed under MIT X11 license (for details please see \doc\license.txt)
+
 using System;
 
 namespace ICSharpCode.NRefactory.TypeSystem.Implementation
diff --git a/ICSharpCode.NRefactory/TypeSystem/Implementation/NullType.cs b/ICSharpCode.NRefactory/TypeSystem/Implementation/NullType.cs
index 3811304387..b1f598c59c 100644
--- a/ICSharpCode.NRefactory/TypeSystem/Implementation/NullType.cs
+++ b/ICSharpCode.NRefactory/TypeSystem/Implementation/NullType.cs
@@ -1,9 +1,5 @@
-// <file>
-//     <copyright see="prj:///doc/copyright.txt"/>
-//     <license see="prj:///doc/license.txt"/>
-//     <author name="Daniel Grunwald"/>
-//     <version>$Revision$</version>
-// </file>
+// Copyright (c) 2010 AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
+// This code is distributed under MIT X11 license (for details please see \doc\license.txt)
 
 using System;
 
diff --git a/ICSharpCode.NRefactory/TypeSystem/Implementation/ProxyTypeResolveContext.cs b/ICSharpCode.NRefactory/TypeSystem/Implementation/ProxyTypeResolveContext.cs
index aacd87c0dc..24fbb205a0 100644
--- a/ICSharpCode.NRefactory/TypeSystem/Implementation/ProxyTypeResolveContext.cs
+++ b/ICSharpCode.NRefactory/TypeSystem/Implementation/ProxyTypeResolveContext.cs
@@ -1,9 +1,6 @@
-// <file>
-//     <copyright see="prj:///doc/copyright.txt"/>
-//     <license see="prj:///doc/license.txt"/>
-//     <author name="Daniel Grunwald"/>
-//     <version>$Revision$</version>
-// </file>
+// Copyright (c) 2010 AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
+// This code is distributed under MIT X11 license (for details please see \doc\license.txt)
+
 using System;
 
 namespace ICSharpCode.NRefactory.TypeSystem.Implementation
diff --git a/ICSharpCode.NRefactory/TypeSystem/Implementation/SimpleConstantValue.cs b/ICSharpCode.NRefactory/TypeSystem/Implementation/SimpleConstantValue.cs
new file mode 100644
index 0000000000..420f904674
--- /dev/null
+++ b/ICSharpCode.NRefactory/TypeSystem/Implementation/SimpleConstantValue.cs
@@ -0,0 +1,37 @@
+// Copyright (c) 2010 AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
+// This code is distributed under MIT X11 license (for details please see \doc\license.txt)
+
+using System;
+
+namespace ICSharpCode.NRefactory.TypeSystem.Implementation
+{
+	/// <summary>
+	/// A simple constant value that is independent of the resolve context.
+	/// </summary>
+	public sealed class SimpleConstantValue : Immutable, IConstantValue
+	{
+		readonly ITypeReference type;
+		readonly object value;
+		
+		public SimpleConstantValue(ITypeReference type, object value)
+		{
+			if (type == null)
+				throw new ArgumentNullException("type");
+			this.type = type;
+			this.value = value;
+		}
+		
+		public IType GetValueType(ITypeResolveContext context)
+		{
+			return type.Resolve(context);
+		}
+		
+		public object GetValue(ITypeResolveContext context)
+		{
+			if (value is ITypeReference)
+				return ((ITypeReference)value).Resolve(context);
+			else
+				return value;
+		}
+	}
+}
diff --git a/ICSharpCode.NRefactory/TypeSystem/Implementation/TypeWithElementType.cs b/ICSharpCode.NRefactory/TypeSystem/Implementation/TypeWithElementType.cs
index 99947c76a3..aea07d8b67 100644
--- a/ICSharpCode.NRefactory/TypeSystem/Implementation/TypeWithElementType.cs
+++ b/ICSharpCode.NRefactory/TypeSystem/Implementation/TypeWithElementType.cs
@@ -1,9 +1,6 @@
-// <file>
-//     <copyright see="prj:///doc/copyright.txt"/>
-//     <license see="prj:///doc/license.txt"/>
-//     <author name="Daniel Grunwald"/>
-//     <version>$Revision$</version>
-// </file>
+// Copyright (c) 2010 AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
+// This code is distributed under MIT X11 license (for details please see \doc\license.txt)
+
 using System;
 
 namespace ICSharpCode.NRefactory.TypeSystem.Implementation
diff --git a/ICSharpCode.NRefactory/TypeSystem/Implementation/UnknownType.cs b/ICSharpCode.NRefactory/TypeSystem/Implementation/UnknownType.cs
index ce899c98ee..6b9dde775c 100644
--- a/ICSharpCode.NRefactory/TypeSystem/Implementation/UnknownType.cs
+++ b/ICSharpCode.NRefactory/TypeSystem/Implementation/UnknownType.cs
@@ -1,9 +1,5 @@
-// <file>
-//     <copyright see="prj:///doc/copyright.txt"/>
-//     <license see="prj:///doc/license.txt"/>
-//     <author name="Daniel Grunwald"/>
-//     <version>$Revision$</version>
-// </file>
+// Copyright (c) 2010 AlphaSierraPapa for the SharpDevelop Team (for details please see \doc\copyright.txt)
+// This code is distributed under MIT X11 license (for details please see \doc\license.txt)
 
 using System;
 
diff --git a/ICSharpCode.NRefactory/TypeSystem/SharedTypes.cs b/ICSharpCode.NRefactory/TypeSystem/SharedTypes.cs
index 4c186ef1c7..2d8ac77d9c 100644
--- a/ICSharpCode.NRefactory/TypeSystem/SharedTypes.cs
+++ b/ICSharpCode.NRefactory/TypeSystem/SharedTypes.cs
@@ -23,8 +23,11 @@ namespace ICSharpCode.NRefactory.TypeSystem
 		[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Security", "CA2104:DoNotDeclareReadOnlyMutableReferenceTypes", Justification = "It's immutable")]
 		public readonly static IType Null = new NullType();
 		
-		// TODO: implement DynamicType
-		public readonly static IType Dynamic = new UnknownType();
+		/// <summary>
+		/// Type representing the C# 'dynamic' type.
+		/// </summary>
+		[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Security", "CA2104:DoNotDeclareReadOnlyMutableReferenceTypes", Justification = "It's immutable")]
+		public readonly static IType Dynamic = new DynamicType();
 		
 		/*
 		 * I'd like to define static instances for common types like