code: boolean partialIsVariable(MultistepExprHolder testee, int lengthToTest)
    {
    	if(1 == lengthToTest)
    	{
    		WalkingIterator wi = (WalkingIterator)testee.m_exprOwner.getExpression();
    		if(wi.getFirstWalker() instanceof FilterExprWalker)
    			return true;
    	}
    	return false;
    }
docs: Check if results of partial reduction will just be a variable, in
  which case, skip it.