anchor.diff:  %% define unit of measure
   -type unit_of_measure() :: octets| cents | seconds | mb | gb.
   
  -%% define validity period of a product
  --type valid_period() :: daily | weekly | monthly | yearly.
  +%% define recurring charge period
  +-type recur_period() :: daily | weekly | monthly | yearly.
   
   -type valid_for() :: {SDT :: integer() | undefined, EDT :: integer() | undefined}.
   
  
positive.code: 		start_date :: pos_integer() | undefined,
  		end_date :: pos_integer() | undefined,
  		type :: price_type() | undefined,
  		period :: valid_period() | undefined,
  		units :: unit_of_measure() | undefined,
  		size :: integer() | undefined,
  		amount :: integer() | undefined,
  
negative.code: 		start_date :: pos_integer() | undefined,
  		end_date :: pos_integer() | undefined,
  		type :: price_type() | undefined,
  		period :: recur_period() | undefined,
  		units :: unit_of_measure() | undefined,
  		size :: integer() | undefined,
  		amount :: integer() | undefined,
  