You're thinking of it wrong, I think. A fireball doesn't have an opposite- but the damage you do with a fireball can be healed with a different weave, providing the opposite to balance it out in the pattern.
Weaves don't need a polar opposite, but they all have something with a...
I'd have to agree with Sorcha. Every weave has an opposite- what can be done, can be undone, and vice versa. The weave may not be a formulaic opposite, but opposite in purpose- Egwene, with her Talents and knowledge, had by this time had enough experience to figure out how to create a weave she...
We must keep in mind that all Aes Sedai learn completely useless weaves for that machine thing, and so learn a certain amount of utility for creating some on the fly.
Egwene Al Vere also had the huge advantage of being able to see the Tower's records sealed to the Flame- the flame of Tar Valon...