In fact, there are two questions. First, for example,

```
{a,b,c,d}::Indices(vector).
tr{#}::Trace.
A::ImplicitIndex(A_{a b}).
B::ImplicitIndex(B_{a b}).
ex:=tr(c A B) tr(A B);
untrace(_);
explicit_indices(_);
```

`A,B`

are not `ImplicitIndex`

, is it a bug? Second, I rewrite the above code using

```
{a,b,c,d}::Indices(vector).
tr{#}::Trace.
{A,B}::ImplicitIndex.
ex:=tr(c A B) tr(A B);
untrace(_);
A::ImplicitIndex(A_{a b}).
B::ImplicitIndex(B_{a b}).
explicit_indices(_);
```

but it doesn't work. The error report is

```
{RuntimeError: Double index pair a inside a single factor found.
At:
<string>(9): <module>
```

Is this another bug?