Subclause 
[tuple] describes the tuple library that provides a tuple type as
the class template 
tuple that can be instantiated with any number
of arguments
.  Each template argument specifies
the type of an element in the 
tuple.  Consequently, tuples are
heterogeneous, fixed-size collections of values
.  An instantiation of 
tuple with
two arguments is similar to an instantiation of 
pair with the same two arguments
.