Prolog Explanation

gistfile1.prolog

   daytime.
clear_skies.

% I want to have empty tables (facts) `daytime` and `clear_skies`

round(earth).
round(ball).

small(mouse).
small(ball).

% I want to have tables  `round` and `small`
% `round` should contain `earth` and `ball`
% `small` should contain `mouse` and `ball`

time_to_play :- daytime, clear_skies.

% I have empty table (fact) `time_to_play` if 
% there are two other tables (facts) `daytime` and `clear_skies`

suitable_for_playing_with(X) :- round(X), small(X).

% I want to have a table `suitable_for_playing_with`
% such that each value in it must be in tables `round` and `small`

does_nick_want_to_play_with(X) :- time_to_play, suitable_for_playing_with(X).

% I want to have a table `does_nick_want_to_play_with`, if there is a table `time_to_play`
% and there each value is also in table `suitable_for_playing_with`

% queries

?- daytime.
% is there a table `daytime`

?- round(cube)
% is there a table `round` that contains value `cube`

?- round(What).
% is there a table `round` and set `What` to it's values

?- does_nick_want_to_play_with(ball).
% is there a table `does_nick_want_to_play_with` containing `ball`

?- does_nick_want_to_play_with(What).
% is there a table `does_nick_want_to_play_with` 
% and give me the values stored in `What`

different(1, 2). different(1, 3).
different(2, 1). different(2, 3).
different(3, 1). different(3, 2).

% there is a table `different` with the appropriate values

solve_for(A, B, C, D, E) :-
    different(B, D), different(B, A),
    different(A, D), different(A, B),
    different(A, C), different(A, E),
    different(C, E), different(C, D).

% create a table `solve_for` such that each field
% `A`, `B`, `C`, `D`, `E`
% is in the `different` table appropriately 

?- solve_for(A, B, C, D, E)
% get the values that belong to `solve_for` table