Will a precipitate form?

Each salt is written as a pair: its metal and its group, such as ("Ba", "Cl") for barium chloride. When two salt solutions are mixed, the partners swap. You will write the swap, check the new pairs against a short list of solids that do not dissolve, and count the mixtures that give a precipitate.

The idea. In a double displacement, AB + CD gives AD + CB. If AD or CB does not dissolve in water, it comes out as a precipitate.

Your tasks.

1. In swap, return the two new pairs, (a[0], b[1]) and (b[0], a[1]).
2. In precipitate, return the new pair that is in insoluble, or None.
3. Count n_precip, the mixtures in mixtures that give a precipitate.