@Nullable
  default PyAstExpression findAssignedValue() {
    PyPsiUtilsCore.assertValid(this);
    return CachedValuesManager.getCachedValue(this,
                                              () -> CachedValueProvider.Result
                                                .create(findAssignedValueInternal(), PsiModificationTracker.MODIFICATION_COUNT));
  }
@Nullable
  default PyAstExpression findAssignedValue()