HHCODE.NORTH is available since version 2.4.0.
HHCODE.NORTH function returns the cell north of the given one at the given resolution.
It will first convert the HHCode to the resolution and then compute the new cell.
The function accepts 3 different types of HHCode and will return the same type for the resulting HHCode.
Since 2.6.0, the resolution is optional. In that case the length of the input is used to determine the resolution.