Sort the cations

You will finish a small program for the cation group scheme. A dictionary gives the group of each cation. A mixture is a list of cations.

The idea. Each group reagent takes out the cations of one group. The cations that remain go on to the next reagent.

Your tasks.

1. Complete group_of(cation). It returns the group number.
2. Complete precipitated(mixture, g). It returns the list of cations in the mixture that belong to group g.
3. Complete left_after(mixture, g). It returns the list of cations in the mixture whose group is higher than g.