Modify the extract-labels procedure

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.

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