anchor.diff:  
   ; PSR COMPLETO
   
  -; Recebe um PSR e retorna T se estiver completo,
  -; i.e. se tiver uma atribuicao para todas as variaveis.
  -; Retorna NIL caso contrario.
  +; Receives a PSR and returns T if it's complete,
  +; i.e. if you have an attribution for all variables.
  +; Returns NIL otherwise.
   
   (defun psr-completo-p (psr)
   	(let
  
positive.code: 
  ; PSR ATRIBUICAO CONSISTENTE
  
  ; Este teste recebe um PSR, uma variavel e um valor,
  ; e retorna 2 valores.
  
  (defun psr-atribuicao-consistente-p (psr var valor)
  	(let
  
negative.code: 
  ; REMOVE ATRIBUICAO
  
  ; This modifier receives a PSR and a variable,
  ; and changes the PSR received, removing any
  ; attribution the variable that has been done previously.
  ; The variable effectively becomes not be assigned.
  ; The return value of this function is not defined.
  
  
  (defun psr-remove-atribuicao! (psr var) 
  