The embryo sac builder

The functional megaspore divides by mitosis in rounds, and the number of nuclei doubles in each round. You will count the nuclei and build the mature embryo sac.

The idea. The embryo sac has 8 nuclei but 7 cells. The central cell holds two of the nuclei.

Your tasks.

1. Finish nuclei_after. It gives the nuclei after a number of rounds.
2. Finish cells. It gives the number of cells in the mature embryo sac, from the dictionary sac.
3. Finish central_cell_chromosomes. It gives the chromosomes in the central cell for a given $2n$.