EXISTENCE
Conv.EXISTENCE : (thm -> thm)
Deduces existence from unique existence.
When applied to a theorem with a unique-existentially quantified
conclusion, EXISTENCE
returns the same theorem with normal
existential quantification over the same variable.
A |- ?!x. p
------------- EXISTENCE
A |- ?x. p
Fails unless the conclusion of the theorem is unique-existentially quantified.