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Einstein's Challenge, also known as the Five Houses problem.

Urban legend has it that this puzzle was written by Einstein and that he 
claimed only 2% of the world's population could solve it. 

THE PUZZLE

There are five houses in five different colours. 
In each house lives a person of a different nationality. 
These five owners drink a certain beverage, smoke a certain brand of
cigarette and keep a certain pet. 
No owners have the same pet, smoke the same brand of smoke or drink
the same drink. 

HINTS

{1}  The Brit lives in the red house. 
{2}  The Swede keeps dogs as pets. 
{3}  The Dane drinks tea. 
{4}  The green house is on the left of the white house. 
{5}  The green house owner drinks coffee. 
{6}  The person who smokes Pall Mall rears birds. 
{7}  The owner of the yellow house smokes Dunhill. 
{8}  The man living in the house right in the center drinks milk. 
{9}  The Norwegian lives in the first house. 
{10} The man who smokes Blend lives next to the one who keeps cats. 
{11} The man who keeps horses lives next to the man who smokes Dunhill. 
{12} The owner who smokes Blue Master drinks beer. 
{13} The German smokes Prince. 
{14} The Norwegian lives next to the blue house. 
{15} The man who smokes Blend has a neighbour who drinks water. 

The question is: WHO OWNS FISH?  
         
Solution can be obtained by running the query:

        all Einstein(house, pets, smokes,drinks,order)


House       1         2       3         4       5 
Nationality Norwegian Dane    Brit      German  Swede 
Color       Yellow    Blue    Red       Green   White 
Pet         Cats      Horses  Birds     Fish    Dog 
Beverage    Water     Tea     Milk      Coffee  Beer 
Cigarettes  Dunhill   Blend   Pall_Mall Prince  Blue_Master 
}
     
Nationality = Brit | Swede | Dane | Norwegian | German 
Color       = Red | Green | Yellow | Blue | White
Cigarettes  = Pall_Mall | Dunhill | Blue_Master | Blend | Prince
Pet         = Dogs | Birds | Cats | Fish | Horses
Beverage    = Tea | Coffee | Milk | Beer | Water
HouseOrder  = First | Second | Third | Fourth | Fifth 

Pets   = Nationality->>Pet
Drinks = Nationality->>Beverage
House  = Nationality->>Color
Smokes = Nationality->>Cigarettes
Order  = HouseOrder->>Nationality 

pred Einstein(house::House, pets::Pets, smokes::Smokes,drinks::Drinks,order::Order) iff
{1}  house(Brit) = Red &
{2}  pets(Swede) = Dogs &     
{3}  drinks(Dane) = Tea &  
{4}  house(x4) = Green & house(x4a) = White & LeftOf(x4,x4a,order) &  
{5}  house(x5) = Green & drinks(x5) = Coffee & 
{6}  smokes(x6) = Pall_Mall & pets(x6) = Birds &
{7}  house(x7) = Yellow & smokes(x7) = Dunhill & 
{8}  drinks(order(Third)) = Milk & 
{9}  order(First) = Norwegian &  
{10} smokes(x10) = Blend & pets(x10a) = Cats & Neighbour(x10,x10a,order) & 
{11} smokes(x11) = Dunhill & pets(x11a) = Horses & Neighbour(x11,x11a,order) &
{12} smokes(x12) = Blue_Master & drinks(x12) = Beer &
{13} smokes(German) = Prince &
{14} house(Norwegian) <> Blue & house(order(Second)) = Blue & 
{15} smokes(x15) = Blend & drinks(x15a) = Water & Neighbour(x15,x15a,order)  

{Helper predicates}
       
pred Neighbour(x::Nationality,y::Nationality,order::Order) iff {y is neighbour of x}
    x <> y &
    order(z1) = x & order(z2) = y & (z1 = z2 + 1 | z1 = z2 - 1) 
           
pred LeftOf(x::Nationality,y::Nationality,order::Order) iff {x is left of y}
    x <> y &
    order(z1) = x & order(z2) = y & z1 = z2 - 1 

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