6. Define a LISP function SCORE->GRADE which takes a single argument, s, and returns a symbol according to the following scheme: s>90 A 73 GRADE 86.3) = =>B+ (SCORE->GRADE 106) => A (SCORE->GRADE -10.1) => F (SCORE->GRADE 59.9) => F (SCORE->GRADE 83) => B+ (SCORE->GRADE 74) => C+ (SCORE->GRADE 67) => D (SCORE->GRADE 87.0) => A- (SCORE->GRADE '(86.3)) => NIL (SCORE->GRADE 'DOG) => NIL