Modified interpreter so that

  a) it accepts "loop" commands to go back to a previous line number.

  b) it accets "label" commands to label a line number for use by "loop".

Thus if the following is at address 20 in the program, one can say

11 label 0

to create a label[0] with value 20.  Then the statement

5000 loop label 0

will cause the program to jump to instruction 20.  Recall that
the leading integer, e.g., 11 or 5000 is a frame number, not
an instruction address.