primo(x) := PRIME(x) 

  agg(x,y) := APPEND(x,[y])

  f(n,r,m) :=   IF( n<=m ,                     ~
                    f( n+1, IF( primo(n) ,     ~
                                agg(r,n) ,     ~
                                r )~
                      , m                 ) ,  ~
                 r  )

primi_fra(n,m) := f(n, [ ] ,m)