<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _validate(self, all_valid_addresses):
 """Validate that all of the dependencies in the graph exist in the given addresses set."""

    for dependency, dependents in iteritems(self._dependent_address_map):
      if dependency not in all_valid_addresses:
        raise AddressLookupError(
            'Dependent graph construction failed: {} did not exist. Was depended on by:\n  {}'.format(
             dependency.spec,
             '\n  '.join(d.spec for d in dependents)
          )
        )