= . command
        .set label to the line to be used by Find,Sort etc command to limit
         the line to apply the command.
        .specify alpha-numeric after ".".

        .ex) sort line 9 to 14

                ==>SOR 1-12 .a .b
               ****|********** TOP
               0001*20040305
               0002*01 23 51 123
               0003*01 23 55 228
               0004*01 24 12 560
               0005*02 13 01 125
               0006*02 43 33 063
               0007*03 03 34 970
               0008*20040307
               .b  *11 23 51 123
               0010*01 13 55 228
               0011*12 24 12 560
               0012*02 13 01 125
               0013*12 43 33 063
               .a  *03 03 34 970
               0015*20040308