Change the names of all the variables

A math student is pestered by a classmate who wants to copy his homework assignment. The student hesitates, not only because he thinks it's wrong, but also because he doesn't want to be sanctioned for aiding and abetting.

His classmate calms him down: "Nobody will be able to trace my homework to you: I'll be changing the names of all the constants and variables: a to b, x to y, and so on."

Not quite convinced, but eager to be left alone, the student hands his completed assignment to the classmate for copying.

After the deadline, the student asks: "Did you really change the names of all the variables?"

"Sure!" the classmate replies. "When you called a function f, I called it g; when you called a variable x, I renamed it to y; and when you were writing about the log of x+1, I called it the timber of x+1¡­"