/* Example showing how backtrack works */

a :- write('a1\n'),b,b.
a :- write('a2\n'),b.
b :- write('b1\n'),c.
b :- write('b2\n'),c.
b :- write('b3\n'),fail.
c :- write('c\n').

/* Test run:

| ?- a.
a1
b1
c
b1
c

true ? ;
b2
c

true ? ;
b3
b2
c
b1
c

true ? ;
b2
c

true ? ;
b3
b3
a2
b1
c

true ? ;
b2
c

true ? ;
b3



Test run with tracing:

| ?- a.
      1    1  Call: a ? 
      3    2  Call: b ? 
      4    3  Call: write('b1\n') ? 
b1
      4    3  Exit: write('b1\n') ? 
      5    3  Call: c ? 
      6    4  Call: write('c\n') ? 
c
      6    4  Exit: write('c\n') ? 
      5    3  Exit: c ? 
      3    2  Exit: b ? 
      7    2  Call: b ? 
      8    3  Call: write('b1\n') ? 
b1
      8    3  Exit: write('b1\n') ? 
      9    3  Call: c ? 
     10    4  Call: write('c\n') ? 
c
     10    4  Exit: write('c\n') ? 
      9    3  Exit: c ? 
      7    2  Exit: b ? 
      1    1  Exit: a ? 

true ? ;
      1    1  Redo: a ? 
      7    2  Redo: b ? 
      8    3  Call: write('b2\n') ? 
b2
      8    3  Exit: write('b2\n') ? 
      9    3  Call: c ? 
     10    4  Call: write('c\n') ? 
c
     10    4  Exit: write('c\n') ? 
      9    3  Exit: c ? 
      7    2  Exit: b ? 
      1    1  Exit: a ? 

true ? 

*/