add "trace *" and introduce a fragment using it to show backtracking

[scpubgit/DX.git] / fragments / btdemo

trace *
?
eq ?X {{ a 1 b 2 c 3 }}
eq ?Y {{ d 1 e 2 f 3 }}
member_at X ?XKey ?XValue
member_at Y ?YKey ?YValue
eq XValue 2
eq YValue 2
...