Exercise 5.8: The following register-machine code
is ambiguous, because the label here is defined more than once:


start
  (goto (label here))
here
  (assign a (const 3))
  (goto (label there))
here
  (assign a (const 4))
  (goto (label there))
there

With the simulator as written, what will the contents of register a be
when control reaches there?  Modify the extract-labels procedure
so that the assembler will signal an error if the same label name is used to
indicate two different locations.