anchor.diff:  -record(product,
   		{name :: string(),
   		description :: string(),
  +		valid_for	:: {integer(), integer()},
   		is_bundle = false :: boolean(),
   		status :: product_status(),
   		start_date :: pos_integer(), % ISO8601
  
positive.code: -type price_type() :: recurring | one_time | usage.
  
  %% define unit of measure
  -type unit_of_measure() :: octet| cents | seconds.
  
  %% define validity period of a product
  -type valid_period() :: daily | weekly | monthly | yearly.
  
negative.code: -record(price,
  		{name :: string(),
  		description :: string(),
  		valid_for	:: {integer(), integer()},
  		type :: price_type(),
  		units :: unit_of_measure(),
  		currency :: string(),
  