NEXTDOWN function consumes a numeric parameter from the top of the stack and pushes back the next DOUBLE in the direction of negative infinity.
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 fonction 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.