The Stacks project

Remove opp two times

Thanks to Eric Ahlqvist
http://stacks.math.columbia.edu/tag/07Y3#comment-2584

Strong formal effectiveness => openness versality

Actually not hard to prove and you only need to check on first order
thickenings.

Bhargav's example from c31011d shows that strong formal effectiveness
does not always hold. But what about strong formal effectiveness
where the thickenings are always first order (between any two, not
just between consecutive indices)? This is the only thing needed for
the argument here, so it would be nice if it was true for algebraic
stacks in general.

Fix typo

Thanks to Ariyan
http://stacks.math.columbia.edu/tag/07WY#comment-1588

Small changes

Tags: Added new tags

First version of Artin's theorems

	This monster commit contains the first version of Artin's
	theorems on representability of stacks and spaces. It is still a
	bit rough.