define

  # --- presents and workers ---

  entity present-blueprint,
    owns name,
    owns description,
	  plays production:blueprint;

  entity christmas-worker,
    owns name,
    plays lives-in:being;

  entity elf,
    sub christmas-worker,
	  owns email @key,
    owns status @values("working", "retired"),
    plays production:builder,
    plays coffee-boost:elf;

  entity santa,
    sub christmas-worker,
    owns name @key @values("Santa"), # trick: key + a single value allowed!
    plays santa-journey:santa;

  relation santa-journey,
    relates santa,
    relates route;
  
  relation production,
    relates builder @card(0..),
    relates blueprint @card(1),
    owns quantity-required @card(1),
    plays present-distribution:source,
    plays coffee-boost:production;
  
  relation lives-in,
    relates location @card(1),
	  relates being @card(1);

  entity coffee,
    owns cups,
    plays coffee-boost:coffee;

  relation coffee-boost,
    relates elf,
    relates production @card(0..),
    relates coffee;

  # --- places and demographics ---

  entity region @abstract,
    owns name,
    plays lives-in:location;

  # Day 9 additions included!
  entity planet,
    sub region,
    plays location-contains:parent;

  entity continent,
    sub region,
    plays location-contains:parent,
    plays location-contains:child;

  entity country,
   	sub region,
    plays location-contains:parent,
    plays location-contains:child,
    plays demographics:place;

  entity city,
    sub region,
    owns is-capital,
    owns city-population,
    plays location-contains:child;

  relation location-contains,
    relates parent,
	  relates child @card(1);

  entity population-statistics,
    owns population,
    plays demographics:statistics @card(1);

  entity city-statistics,
    sub population-statistics;
  
  entity country-statistics,
    sub population-statistics,
    owns proportion-under-12;

  relation demographics,
    relates place,
    relates statistics;

  # --- travel plans ---

  # santa needs to plot a route to each country in the world
  entity santa-distribution-route,
    owns start-date @key,
    plays distribution-stop:route,
    plays santa-journey:route;

  relation distribution-stop,
    relates country @card(1),
    relates route @card(1),
    owns stop-number @card(1),
    plays present-distribution:distribution-stop; # NOTE: not going to get used, too much complexity
  
  entity country, plays distribution-stop:country;

  # NOTE: not going to get used, too much complexity.
  relation present-distribution,
    relates distribution-stop @card(1),
    relates source @card(1);

  attribute start-date, value datetime;
  attribute is-capital, value boolean;
  attribute status, value string;
  attribute name, value string;
  attribute description, value string;
  attribute email, value string;
  attribute quantity-required, value integer;

  attribute population, value integer;
  attribute proportion-under-12, value double;
  attribute city-population, value integer;
  attribute stop-number, value integer;

  attribute cups, value integer;