Build an alkane name

You will write three functions that build the name of an alkane with methyl groups on its chain.

The idea. The word root comes from the length of the chain. Number the chain from both ends and keep the set of locants that is lower at the first difference. Then write the locants, a multiplier (di, tri), the word methyl, and the parent name.

Your tasks.

1. In stem, return the word root for a chain of n carbons.
2. In lowest, return the lower set of locants, as a sorted list.
3. In name_methyl, return the whole name, such as 2,3-dimethylpentane. With no branches, return the plain alkane name.