<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def inject_target(self, target, dependencies=None, derived_from=None, synthetic=False):
 """Injects a fully realized Target into the BuildGraph. :API: public :param Target target: The Target to inject. :param list<Address> dependencies: The Target addresses that `target` depends on. :param Target derived_from: The Target that `target` was derived from, usually as a result of codegen. :param bool synthetic: Whether to flag this target as synthetic, even if it isn't derived from another target. """

    if self.contains_address(target.address):
      raise ValueError('Attempted to inject synthetic {target} derived from {derived_from}'
                       ' into the BuildGraph with address {address}, but there is already a Target'
                       ' {existing_target} with that address'
                       .format(target=target,
                               derived_from=derived_from,
                               address=target.address,
                               existing_target=self.get_target(target.address)))

    dependencies = dependencies or frozenset()
    address = target.address

    if address in self._target_by_address:
      raise ValueError('A Target {existing_target} already exists in the BuildGraph at address'
                       ' {address}.  Failed to insert {target}.'
                       .format(existing_target=self._target_by_address[address],
                               address=address,
                               target=target))

    if derived_from:
      if not self.contains_address(derived_from.address):
        raise ValueError('Attempted to inject synthetic {target} derived from {derived_from}'
                         ' into the BuildGraph, but {derived_from} was not in the BuildGraph.'
                         ' Synthetic Targets must be derived from no Target (None) or from a'
                         ' Target already in the BuildGraph.'
                         .format(target=target,
                                 derived_from=derived_from))
      self._derived_from_by_derivative[target.address] = derived_from.address
      self._derivatives_by_derived_from[derived_from.address].append(target.address)

    if derived_from or synthetic:
      self.synthetic_addresses.add(address)

    self._target_by_address[address] = target

    for dependency_address in dependencies:
      self.inject_dependency(dependent=address, dependency=dependency_address)