The C2a standard added the em(three-way comparison) operator tt(<=>), also
known as the em(spaceship operator), to bf(C++). In bf(C++) operators can be
defined for class-types, among which equality and comparison operators (the
familiar set of tt(==, !=, <, <=, >) and tt(>=) operators).  To provide
classes with all comparison operators merely the equality and the spaceship
operator need to be defined.

Its priority hi(<=>: priority) is less than the priorities of the bit-shift
operators tt(<<) and tt(>>) and larger than the priorities of the ordering
operators tt(<, <=, >,) and tt(>=).

Section ref(SPACESHIP) covers the construction of the three-way comparison
operator.

