public static AddBuiltinProxy <TSettings> AddBuiltin <TSettings>(this IBuiltinFunctionWritableCompilerSettings <TSettings> self)
 => new AddBuiltinProxy <TSettings>(self);
 internal AddBuiltinProxy(IBuiltinFunctionWritableCompilerSettings <TSettings> s)
 => settings = s;
 public static void AddBuiltin <TSettings>(this IBuiltinFunctionWritableCompilerSettings <TSettings> self, IBuiltinFunction <TSettings> function)
 {
     self.WritableBuiltinFunctions.Add(function);
 }