public FieldDefinition InsertField (FieldDefinition field, bool clone) {
			if (fields.ContainsKey (field.Name)) {
				Console.Error.WriteLine ("Error: duplicated field {0}", field.Name);
				return null;
			} else {
				if (clone) {
					field = field.Clone ();
				}
				fields [field.Name] = field;
				return field;
			}
		}