I already shared the ideas for this entry with JoeJ. This is a short recap of them.

At any given time when the user has to take an economy decision, his decision to build something means that he decides to omit to build a number of other things. Basically there is a cloud of related options at any given time. I call this cloud 'frame'. They are related in the sense that they are specific to a certain stage of the game. Seen as a timeline perspective the game appears as a tree of frames.