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http://www.tfd.com/constructive
1. Serving to improve or advance; helpful: constructive criticism.
2. Of or relating to construction; structural.
3. Law Based on an interpretation; not directly expressed.
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http://www.tfd.com/positive
1. Characterized by or displaying certainty, acceptance, or affirmation: a positive answer; positive criticism.
2. Measured or moving forward or in a direction of increase or progress.
3. Explicitly or openly expressed or laid down: a positive demand.
4. Admitting of no doubt; irrefutable: positive proof.
5.
a. Very sure; confident: I'm positive he's right. See Synonyms at sure.
b. Overconfident; dogmatic.
6. Formally or arbitrarily determined; prescribed.
7. Concerned with practical rather than theoretical matters.
8. Composed of or characterized by the presence of particular qualities or attributes; real.