Which group does the reagent reduce?

You will write a small program that decides what a reagent does to a molecule that has several groups. It also counts the hydride ions that a reduction needs.

The idea. Each reagent reduces a fixed set of groups. A molecule is a list of its groups. The reagent reduces the groups that it can, and leaves the others.

Your tasks.

1. Complete reduced(reagent, molecule) so that it returns the groups of the molecule that the reagent reduces.
2. Complete left_alone(reagent, molecule) so that it returns the groups that the reagent leaves.
3. Complete hydrides(molecule) so that it adds up the hydride ions needed: ketone 1, aldehyde 1, ester 2.