ATAN function consumes a floating point number between -1 et 1 from the top of the stack and pushes back its arctangent in radians.
Since 2.6.0, this function can be applied to a list of numeric values. In this case, the function is applied to each element of the list thus returning a list.
Since 2.7.1, this function can be also applied to a GTS. In this case, the function is applied to each value of the given GTS, returning a new GTS.