fails:Float#arg returns NaN if NaN
fails:Float#arg returns self if NaN
fails:Float#arg returns 0 if positive
fails:Float#arg returns 0 if +0.0
fails:Float#arg returns 0 if +Infinity
fails:Float#arg returns Pi if negative
fails:Float#arg returns Pi if -0.0
fails:Float#arg returns Pi if -Infinity
