fig18_2.pl

% Figure 18.2  A planning domain specification for the robots-on-grid world of Figure 18.1.

% A planning domain: robots a, b and c move in grid 3 by 2

:-  op( 100, fx, ~).		% Operator for negated conditions

% Precondition of action: robot R goes from cell A to B

can( go(R,A,B), [ at(R,A), clear(B)])  :-	
  robot(R), adjacent(A,B).

%   Effects of action go( Robot, CellA, CellB):
%      After the action at(Robot,CellB) and clear(CellA) become true, 
%      and at(Robot,CellA) and clear(CellB) become false

effects( go(R,A,B), [ at(R,B), clear(A), ~at(R,A), ~clear(B)]).

robot(a).    robot(b).   robot(c).

adjacent(A,B)  :-		% Cells A and B are adjacent in the grid
  n(A,B); n(B,A).

%  n( A, B):  Cells A and B are “ordered” neighbours

n(1,2).  n(2,3).  n(4,5).  n(5,6).  n(1,4).  n(2,5).  n(3,6).

% inconsistent( G1, G2): goals G1 and G2 inconsistent

inconsistent( G, ~G).			% Negated goals always inconsistent	
inconsistent( ~G, G).

inconsistent( at(R,C1), at(R,C2))  :-
  C1 \== C2.    	 		% Robot cannot be at different places at the same time

inconsistent( at(_,C), clear(C)).	% A cell cannot be both clear and occupied at a time
inconsistent( clear(C), at(_,C)).

inconsistent( at(R1,C), at(R2,C))  :-   % Two robots cannot be in the same cell at a time
  R1 \== R2.	

state0( [ at(a,1), at(b,2), at(c,3), clear(4), clear(5), clear(6)]).	% The state of Fig. 18.1