docs: def lookup(self, opaque_ref):
          """
          Lookups the object that was used for creating the reference.
          """
          opaque_ref_value = self._get_reference_value(opaque_ref)
          # check local refs
          if opaque_ref_value in self.local_refs:
              return self.local_refs[opaque_ref_value]
          # check global refs
          if opaque_ref_value in self.global_refs:
              return self.global_refs[opaque_ref_value]
          raise KeyError("Unknown JNI reference %d. Local references: %s Global references: %s"
                         % (opaque_ref_value, self.local_refs, self.global_refs))