Used to construct a Tuple with the parameters as its values.

Calling tuple() produces a hint to make a Tuple but does not actually construct Tuple, as the types in the Tuple are not known yet here and are not assumed to be the same as the parameters.

The marker type holds a refernce to each parameter, allowing it to construct a Tuple which holds references correctly. The marker must be consumed immediately into a Tuple variable, which is normally done by using the tuple function as a function argument or in a return statement.