anchor.diff:  
   (defn {sum l} { reduce + 0 l })
   (defn {product l} { reduce * 1 l })
  +
  +; recursive variant, sums trees
  +(defn {sum2 l} {
  +  if (num? l)
  +    {l}
  +    {apply + (map sum2 l)}
  +})
   
   (defn {filter f l} {
     if (empty? l)
  
positive.code:     {nil}
      {join (list (join (head x) (head y))) (zip (tail x) (tail y))}
  })
  
negative.code: (defn {even? x} { not (odd? x) })
  
  (defn {num? x} {
    eq "number" (type x)
  })
  
  (defn {str? x} {
    eq "string" (type x)
  })
  
  (defn {list? x} {
    in? {"sexpr" "qexpr"} (type x)
  })
  
  ;; logical ops
  