For racket language

1. Write a structurally recursive function (positions-of s los) that takes two arguments, a symbol s and a list of symbols los.

     ::= () | ( . ) 

   positions-of returns a list containing the zero-based positions of all occurrences of s in los. For example:

    > (positions-of 'a '(a b a c d a e f g a h i j k)) '(0 2 5 9) 

   Make positions-of an interface procedure that calls a structurally recursive helper function with the symbol and list of symbols as the first two arguments, and an initial value for the counter as the third.