When opening mail you should:

keep the envelope
compare date of letter with postmark
date stamp the letter
all the above

i need with the same thing

. a function is a relation in which each input has only one output. in the relation , y is a function of x, because for each input x (1, 2, 3, or 0), there is only one output y. x is not a function of y, because the input y = 3 has multiple outputs: x = 1 and x = 2.

the author wants to draw the reader deeper into the experiences.