A peptide toolkit

You will write small functions that treat a peptide as a string of one-letter codes. They count the peptide bonds, find the molar mass, test for the biuret colour and count the different sequences that a set of residues can make.

The idea. A chain of $n$ residues has $n - 1$ peptide bonds, and each bond releases one molecule of water. The biuret test needs at least two peptide bonds. Repeated residues make some orders the same.

Your tasks.

1. Complete bonds(seq).
2. Complete peptide_mass(seq). Use the mass table and subtract one water for each bond.
3. Complete biuret(seq) so that it returns True when the peptide has at least two peptide bonds.
4. Complete orders(seq) so that it returns the number of different sequences that can be made from the letters of seq.