Sentence:  The <WSD>grue</WSD> property is defined as: "x" is grue if and only if "x" is green and is observed before the year 2000, or "x" is blue and is not observed before the year 2000. 
Meaning of the word marked with <WSD> tags:  Of an object, green when first observed before a specified time or blue when first observed after that time.