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sqrt(x) := if x1 >= R 0R then u+vi else u+(-Rv)i where u = sqrt((x0 +R sqrt(x02 +R x12)) /R 2R) v = sqrt((-Rx0 +R sqrt(x02 +R x12)) /R 2R).
Proposition: the (positive or negative) root of x squared equals x.
forall x in C : let r = sqrt(x): x=r*r /\ x=(-r)*(-r).