protected static void operationInputOutputDefinition(CapturingTokenizer tokenizer) {
        if (tokenizer.expect(IdlToken.COLON, IdlToken.WALRUS) == IdlToken.COLON) {
            tokenizer.next();
            optionalWs(tokenizer);
            SHAPE_ID.parse(tokenizer);
        } else {
            INLINE_AGGREGATE_SHAPE.parse(tokenizer);
        }
    }
The innermost node of the token tree that contains an actual token returned from {@link IdlTokenizer}.
smithy-syntax/src/main/java/software/amazon/smithy/syntax/TreeType.java