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CHR/chr/examples/path.pl

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+% PATH CONSISTENCY, simple
+% thom fruehwirth ECRC 941201, simplified version of time-pc.chr
+% 980311 Thom Fruehwirth, LMU, adapted to Sicstus CHR
+
+:- use_module(library(chr)).
+
+handler path.
+
+option(already_in_heads,on).
+
+constraints con/3.
+% con(X,Y,C) means that constraint C holds between variables X and Y
+
+intersect_xy_xy @ con(X, Y, C1), con(X, Y, C2) <=> 
+	inter(C1, C2, C3),
+	con(X,Y,C3).
+intersect_xy_yx @ con(X, Y, C1), con(Y, X, CR) <=> 
+	invert(CR, C2),  
+	inter(C1, C2, C3),
+	con(X,Y,C3).
+
+propagate_xy_yz @ con(X, Y, C1), con(Y, Z, C2) ==>
+	trans(C1, C2, C3)
+ 	| 
+	con(X, Z, C3).
+propagate_xy_xz @ con(X, Y, CR), con(X, Z, C2) ==>
+	invert(CR,C1),
+	trans(C1, C2, C3)
+ 	|
+	con(Y, Z, C3).
+propagate_xy_zy @ con(X, Y, C1), con(Z, Y, CR) ==>
+	invert(CR,C2),
+	trans(C1, C2, C3)
+ 	| 
+	con(X, Z, C3).
+
+
+% Example ---------------------------------
+% constraints are < and >
+
+invert(<,>).
+invert(>,<).
+
+% fail if empty constraint would be produced
+inter(C,C,C).
+
+% fail if most general constraint would be produced
+trans(C,C,C).
+
+% ?- con(A,B,>),con(A,C,>),con(B,D,>),con(C,D,>).
+
+/*--------------- eof path.pl ----------------------------------------------*/