--------------------------------------------------------------------
  --------------------------------------------------------------------
  -- Function auxiliary operations
  --------------------------------------------------------------------
  --------------------------------------------------------------------
  -- Function is a set {(x1, y1),...,(xn, yn)}
  
  -- Transforms a singleton set into the element itself
  pick({x}) = x
  
  -- Returns the function return
  -- Raises error if z is not in the domain of the function
  apply(<(x,y)>^rs,z) =
  	if x==z then y else apply(rs,z)
  
  -- domain antirestriction
  ddres(f,xs) = <(n,v) | (n,v) <- f, not member(n,xs)>
  
  -- domain restriction
  dres(f,xs) = <(n,v) | (n,v) <- f, member(n,xs)>
  
  -- Overwrites the fuction
  over(<>,n,v) = <>
  over(<(x,y)>^rs,n,v) =
  	if x==n then <(x,v)>^rs else <(x,y)>^over(rs,n,v)
  
  -- Returns the domain of a relation
  dom(f) = set(<n | (n,v) <- f>)
  
  -- Returns the domain of a relation
  ran(f) = set(<v | (n,v) <- f>)
  
  -- Subset or equals
  subseteq(S,T) = diff(S,T) == {}