Naming the spots of a chromatogram

You will write two small functions for a TLC plate. One finds the $R_f$ of a spot. The other names a spot by matching its $R_f$ with known dyes run on the same plate.

The idea. $R_f$ is the distance moved by the spot divided by the distance moved by the solvent front. Two spots with $R_f$ values within a small tolerance are taken to be the same compound.

Your tasks.

1. Complete rf(distance, front) so that it returns the $R_f$.
2. Complete identify(rf_value, known, tolerance): return the name of the known dye whose $R_f$ is within tolerance of rf_value, or "unknown".
3. Set result to a list with the name for each spot, in the order of the dictionary.