Sentence:  From a constitutional law standpoint, the often debated question arises as to whether the objective net principle can be sufficiently met simply by way of a chronological loss netting within a given source of income alone or whether the rule of equality of taxation requires that losses be <WSD>nettable</WSD> vertically in principle except insofar as particular ground for justification, such as averting abusive tax planning, undermine the principle of setting off losses. 
Meaning of the word marked with <WSD> tags:  Capable of producing a net profit on the books.