% Figure 2.8

vertical( seg( point( X, Y), point( X, Y1) ) ).
horizontal( seg( point( X, Y), point( X1, Y) ) ).

% Figure 2.10

big( bear).      % Clause 1
big( elephant).  % Clause 2
small( cat).     % Clause 3

brown( bear).    % Clause 4
black( cat).     % Clause 5
gray( elephant). % Clause 6
dark( Z) :-
  black( Z).     % Clause 7: Anything black is dark
dark( Z) :-
  brown( Z).     % Clause 8: Anything brown is dark

% Four versions of the ancestor program

% The original version

anc1( X, Z) :-
  parent( X, Z).

anc1( X, Z) :-
  parent( X, Y),
  anc1( Y, Z).

% Variation 1: swap clauses of the original version

anc2( X, Z) :-
  parent( X, Y),
  anc2( Y, Z).

anc2( X, Z) :-
  parent( X, Z).

% Variation 2: swap goals in second clause of the original version

anc3( X, Z) :-
  parent( X, Z).

anc3( X, Z) :-
  anc3( X, Y),
  parent( Y, Z).

% Variation 3: swap goals and clauses of the original version

anc4( X, Z) :-
  anc4( X, Y),
  parent( Y, Z).

anc4( X, Z) :-
  parent( X, Z).