Improvements on goal oriented strategies

Explore a way to have more control on the flow of execution of goal oriented strategies. One thing that would be interesting would be to choose the next strat to apply based on the result of a strategy, ie. dynamically generate an order of execution. For instance, we could apply a transformation that generates a list of subtasks, and then instead of applying strats to the subtasks in the subtasks creation order, the order is specified by the strategy.