Consider:

```text
                root
                 |
                 v
            intermediate-1
                 |
                 v
            intermediate-2
                 |
                 v
            intermediate-3
       /    |    |    |    \    trust amount: 1, except for carol
      v     v    v    v     v
    alice  bob carol  ed  dave
       \    |    |    |    /
        v   v    v    v   v
            intermediate-a
                 |
                 v
            intermediate-b
                 |
                 v
            intermediate-c
                 |
                 v
               target
```

Where intermediate-3 designates alice, bob, carol, ed and dave as
trusted introducers with a trust amount of 1 except for carol.  When
searching for a path, we need the path via carol.
