E1:     C1 CALL pred undo.main/2-0 (det) undo.m:18
mdb> mdb> Contexts will not be printed.
mdb> echo on
Command echo enabled.
mdb> step
      E2:     C2 CALL pred undo.sum/2-0 (det)
mdb> finish
      E3:     C2 EXIT pred undo.sum/2-0 (det)
mdb> dd
sum([1, 2, 3, 4, 5, 6], 21)
Valid? no
sum([2, 3, 4, 5, 6], 20)
Valid? undo
sum([1, 2, 3, 4, 5, 6], 21)
Valid? [no] undo
Undo stack empty.
sum([1, 2, 3, 4, 5, 6], 21)
Valid? [no] 
sum([2, 3, 4, 5, 6], 20)
Valid? no
sum([3, 4, 5, 6], 18)
Valid? 
sum([4, 5, 6], 15)
Valid? undo
sum([3, 4, 5, 6], 18)
Valid? 
sum([4, 5, 6], 15)
Valid? no
sum([5, 6], 11)
Valid? no
sum([6], 6)
Valid? pd
      E4:     C3 EXIT pred undo.sum/2-0 (det)
mdb> dd -r
sum([6], 6)
Valid? undo
sum([5, 6], 11)
Valid? [no] undo
sum([4, 5, 6], 15)
Valid? [no] undo
sum([3, 4, 5, 6], 18)
Valid? undo
sum([2, 3, 4, 5, 6], 20)
Valid? [no] undo
sum([1, 2, 3, 4, 5, 6], 21)
Valid? [no] undo
Undo stack empty.
sum([1, 2, 3, 4, 5, 6], 21)
Valid? [no] q
Diagnosis aborted.
      E4:     C3 EXIT pred undo.sum/2-0 (det)
mdb> quit -y