Section 13(b)(12) exempts ``any employee employed in * * *.'' It is
clear from this language that it is the activities of the employee
rather than those of his employer which ultimately determine the
application of the exemption. Thus the exemption may not apply to some
employees of an employer engaged almost exclusively in activities within
the exemption, and it may apply to some employees of an employer engaged
almost exclusively in other activities. But the burden of effecting
segregation between exempt and nonexempt work as between different
groups of employees is upon the employer.