description needs improvement

I am not yet sure how, but the description needs improvement. If I knew, I'd have made a PR on github.com

First of all, the mission is closely related to homomorphism, but it is not the correct word to describe the task. The way I understand it "ff" is homomorph to "barbar", f stands for bar. Secondly, the last paragraph is confusing in precisely this context: "One character from one string can correspond only to one character from another string. Two or more characters of one string can correspond to one character of another string, but not vice versa." The way I understand the example tests, the f->bar replacement isn't considered since both strings are the same size/length.

The tests are for the strings to be isomorph, isometric and bijective.