The star alliance website (www.staralliance.com) has a great planner, which will price the journey for you if you proceed all the way though.
Limited backtracking may be allowed, e.g. SIN
can be done in either direction, if one is flying from west to east, but the planner will be able to tell you if this is allowed or not for your specific route.
The *A ticket is also much more flexible than the oneworld equivalent, as transits DO
NOT count as a city visited, with OW
you can only visit each city once, so LHR
would count even if it was a transit, you could not go though HKG
again on that ticket.
Hope this helps,
Next flights: MAN-ORK-LHR(EI)-MAN(BD); MAN-LHR(BD)-ORK (EI); DUB-ZRH-LAX (LX) LAX-YYZ (AC) YYZ-YHZ-LHR(AC)-DUB(BD)