573: Mainline

Note that the ampersand in the return-value type declaration cannot appear without the ampersand in the parameter type declaration:

• The parameter whose value is the ostream object is a call-by-reference parameter. Thus, nothing is pushed onto the stack for that parameter.
• Because the memory reserved for a call-by-reference parameter does not go on the stack, that memory is not disturbed when the function returns. Thus, there is no need to copy the class object when the function returns. Hence, a copy-free return is possible.