*        Duplicate a string
         MVC    A,=CL64'Hello'     a='Hello'
         MVC    B,A                b=a          memory copy
         MVC    A,=CL64'Goodbye'   a='Goodbye'
         XPRNT  A,L'A              print a
         XPRNT  B,L'B              print b
         ...
*        Make reference to a string a string
         MVC    A,=CL64'Hi!'       a='Hi!'
         LA     R1,A               r1=@a        set pointer
         ST     R1,REFA            refa=@a      store pointer
         XPRNT  A,L'A              print a
         XPRNT  0(R1),L'A          print %refa
         ...
A        DS     CL64               a
B        DS     CL64               b
REFA     DS     A                  @a