Handout 2 qestion 1 - pie function definition

i didnt understand why the pi(f) function is one to one
there is no definition to f whether it’s ont or one by one so ther can be two functions with the same image (linke in the attaches image)

can you give further elaboration a bout the pie function?

please post a complete question. I cannot find Q1 in handout 2… (Moodle HW 2 is about topological sorting…)