$ #trace +step +search
$ ?
? eq ?X {{ a 1 b 2 c 3 }}
SetValue 0.X {{ a 1 b 2 c 3 }}
? member_at X ?Y ?Z
SetValue 0.Y 'a'
BindValue 0.Z 0.X.a
? ...
{{ X {{ a 1 b 2 c 3 }} Y 'a' Z 1 }}
{{ X {{ a 1 b 2 c 3 }} Y 'b' Z 2 }}
{{ X {{ a 1 b 2 c 3 }} Y 'c' Z 3 }}
No next solution
$ qlist
eq ?X {{ a 1 b 2 c 3 }}
member_at X ?Y ?Z
$ ?
? eq ?X {{ a 1 b 2 c 3 }}
SetValue 0.X {{ a 1 b 2 c 3 }}
? member_at X ?Y ?Z
SetValue 0.Y 'a'
BindValue 0.Z 0.X.a
? eq Z 2
? ...
{{ X {{ a 1 b 2 c 3 }} Y 'b' Z 2 }}
No next solution
$ qlist
eq ?X {{ a 1 b 2 c 3 }}
member_at X ?Y ?Z
eq Z 2