anchor.diff:  (defn {even? x} { not (odd? x) })
   
   (defn {num? x} {
  -  (eq "number" (type x))
  +  eq "number" (type x)
   })
   
   (defn {str? x} {
  -  (eq "string" (type x))
  +  eq "string" (type x)
   })
   
   (defn {list? x} {
  -  (in? {"sexpr" "qexpr"} (type x))
  +  in? {"sexpr" "qexpr"} (type x)
   })
   
   ;; logical ops
  
positive.code: 
  (defn {sum l} { reduce + 0 l })
  (defn {product l} { reduce * 1 l })
  
  (defn {filter f l} {
    if (empty? l)
  
negative.code:       {list l}}
  })
  
  (defn {range start end} {
    if (> start end)
      {nil}
      {join (list start) (range (inc start) end)}
  })
  
  ;; functional
  
  (defn {apply f xs} {
  