anchor.diff:  
   type NullableTypeContext(converter : IValueConverter) =
       interface ITypeContext with
  +        member this.GetNamedTypes = upcast []
           member this.GetTranslator(ty) =
               if ty.IsValueType
                   && ty.IsGenericType
  
positive.code: ﻿namespace GraphQL.Parser
  open System.Reflection
  open System.Collections.Generic
  
  
negative.code:     let mutable context : ITypeContext = Unchecked.defaultof<ITypeContext>
      do
          context <-
              customContext(this :> IValueConverter) // custom types get first precedence
              |??| new BuiltinTypeContext()
              |??| new NullableTypeContext(this :> IValueConverter)
              |??| new ArrayTypeContext(this :> IValueConverter)
              |??| new CollectionTypeContext(this :> IValueConverter)
              |??| new EnumerableTypeContext(this :> IValueConverter)
              |??| new SingleConstructorTypeContext(this :> IValueConverter)
      interface IValueConverter with
          member this.GetNamedTypes = context.GetNamedTypes
          // TODO: cache translators for CLR types
          member this.VariableTypeOf(targetType) =
              match context.GetTranslator(targetType) with
  