question on BGP aggregation


I observe some BGP AS paths collected from Routeview having the AS-set
in the last hop. According to my understanding, this is BGP route
aggregation. However, my question is as follows:
Suppose, there is a path AS1 AS2 AS3 {AS4 AS5 AS6}, how AS4 AS5 AS6
connect to AS3?
Does it necessarily mean that AS4 AS5 AS6 are direct neighbors of AS4,
and AS4 aggregate the routes from AS4 AS5 AS6 then exporting outside.
or, it could be other cases such as AS4 aggregate AS5 AS6 first, and
then AS3 aggregate AS4?

Thanks in advance,

the ASes in the AS_SET resulted from merging 2 or more AS_PATHS, you only know at least one of them is connected to AS3 ...
more details at rfc4271:

"An AS_SET implies that the destinations listed in the NLRI can
          be reached through paths that traverse at least some of the
          constituent autonomous systems. AS_SETs provide sufficient
          information to avoid routing information looping; however,
          their use may prune potentially feasible paths because such
          paths are no longer listed individually in the form of
          AS_SEQUENCEs. In practice, this is not likely to be a problem
          because once an IP packet arrives at the edge of a group of
          autonomous systems, the BGP speaker is likely to have more
          detailed path information and can distinguish individual paths
          from destinations.


A further question, is it mandatory for all the aggregated information
be appended at the AS path, is it possible for some aggregations do
not propagate outside? By this, I mean some ASes completely conceal
their aggregated ASes when propagation.

thanks a lot.