fact: Definition atan (x : R) : R := get [set y | -(pi / 2) < y < pi / 2 /\ tan y = x].
imports: mathcomp(all_ssreflect ssralg ssrint ssrnum matrix) mathcomp(interval rat) mathcomp(mathcomp_extra boolp classical_sets functions) reals ereal nsatz_realtype signed topology normedtype landau sequences derive realfun exp