fun isRelatedTo(a, b, []) = false 
   | isRelatedTo(a, b, (h1, h2)::rest) = 
            (a = h1 andalso b = h2) orelse isRelatedTo(a, b, rest);

datatype city = Vancouver | LosAngeles | Mexico | Denver | Omaha |
         KansasCity | Minneapolis | StLouis | Memphis | NewOrleans |
         Chicago | Cincinnati | Pittsburgh | Montreal | NewYork;

datatype river = Missouri | Platte | NPlatte | SPlatte | Arkansas | Canadian | Kansas |
         Mississippi | Tennessee | Ohio | Allegheny | Monongahela;

val isOnRiver = [(Denver, SPlatte), (Omaha, Missouri), (Omaha, Platte),
                 (KansasCity, Missouri), (KansasCity, Kansas), (Minneapolis, Mississippi),
                 (StLouis, Mississippi), (StLouis, Missouri), (Memphis, Mississippi),
                 (NewOrleans, Mississippi), (Cincinnati, Ohio),
                 (Pittsburgh, Ohio), (Pittsburgh, Allegheny), (Pittsburgh, Monongahela)];

val flowsInto = [(Platte, Missouri), (Kansas, Missouri), (Missouri, Mississippi),
                 (Canadian, Arkansas), (Arkansas, Mississippi),
                 (Allegheny, Ohio), (Monongahela, Ohio), (Tennessee, Ohio),
                 (Ohio, Mississippi), (NPlatte, Platte), (SPlatte, Platte)];

datatype Tribe = Asher | Naphtali | Zebulun | Issachar | Dan 
               | Gad | Manasseh | Reuben | Ephraim | Benjamin 
               | Judah | Simeon;


 datatype WaterBody = Mediterranean | Dead | Jordan 
                    | Chinnereth | Arnon | Jabbok;

val tribeBordersWater =
 [(Asher, Mediterranean), (Manasseh, Mediterranean), (Ephraim, Mediterranean),
  (Naphtali, Chinnereth), (Naphtali, Jordan), (Issachar, Jordan), 
  (Manasseh, Jordan), (Gad, Jordan), (Benjamin, Jordan), (Judah, Jordan), 
  (Reuben, Jordan), (Judah, Dead), (Reuben, Dead), (Simeon, Dead),
  (Gad, Jabbok), (Reuben, Jabbok), (Reuben, Arnon)];

val waterImmedWestOf = 
 [(Mediterranean, Chinnereth), 
  (Mediterranean, Jordan), (Mediterranean, Dead),
  (Chinnereth, Jabbok), (Chinnereth, Arnon), 
  (Jordan, Jabbok), (Jordan, Arnon), 
  (Dead, Jabbok), (Dead, Arnon)];