“As I have already plainly said, Starswirl's spells were quite ingenious, in that their design encouraged modification. In common parlance, some unicorns refer to the layers of the corona, although I stress that those are laypony's terms. So, to put it simply, Starswirl's spells were layered in such a way that each separate effect had its own layer, and it could be changed by the caster at will, without requiring the complex effort of writing a spell from scratch.”