In Racket form please!!!

1. Define collect-prop-variables, which will take as input a list representing a proposition, and return a list, representing a set, of all the variables used in the expression. Example: (collect-prop-variables '(A and (not A)) ) should return '(A) and (collect-prop-variables '((B iff (A or C)) implies (not (C implies A ) )) ) should return '(A B C)