I'm learning to write mathematical proofs. When the statement to be proven is in the form "p if and only if q", the proof is often broken into two parts: necessity and sufficiency. I wonder whether I should organize my proof like:
... or vice versa?
Since p<=>q is is equivalent to q<=>p, does it really matter? Is there any accepted practise to put p=>q in necessity or sufficiency, depending on the order in which the statements are presented?