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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def dependencies_of(self, address):
 """Returns the dependencies of the Target at `address`. This method asserts that the address given is actually in the BuildGraph. :API: public """

    assert address in self._target_by_address, (
      'Cannot retrieve dependencies of {address} because it is not in the BuildGraph.'
      .format(address=address)
    )
    return self._target_dependencies_by_address[address]