Return the principal value of the arc tangent of {expr}, in the range [-pi/2, +pi/2] radians, as a |Float|. {expr} must evaluate to a |Float| or a |Number|. Returns 0.0 if {expr} is not a |Float| or a |Number|. Examples: :echo atan(100) 1.560797 :echo atan(-4.01) -1.326405 Can also be used as a |method|: Compute()->atan() {only available when compiled with the |+float| feature}