POWERS(EXPR,VAR)
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THIS FUNCTION IS A GENERALISATION OF "HIPOW" AND "LOPOW"
IN THAT IT RETURNS A LIST OF ALL THE POWERS OF VAR OCCURING
IN EXPR. IT IS STILL NECESSARY TO EXPAND EXPR BEFORE APPLYING
POWERS (ON PAIN OF GETTING THE WRONG ANSWER).

THIS FUNCTION HAS MANY USES, E.G. IF YOU WANT TO FIND ALL
THE COEFFICIENTS OF X IN A POLYNOMIAL POLY YOU CAN USE
MAP(LAMBDA([POW],COEFF(POLY,X,POW)),POWERS(POLY,X));
AND MANY OTHER SIMILAR USEFUL HACKS.
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