Define a LISP function SCORE->GRADE which takes a single argument, s, and returns a symbol according to the following scheme: s90 is A 87s < 90 is A- 83 s < 87 B+ 80 s < 83 B 77 s < 80 B 73 s < 77 C+ 70 s < 73 C 60 s < 70 D s < 60 F

If the argument is not a number then return NIL. Examples of results:

(SCORE->GRADE 59.9) => F (SCORE->GRADE 67) => D (SCORE->GRADE '(86.3)) => NIL (SCORE->GRADE 'DOG) => NIL