Struggling with Orca's macro capabilities: Custom variables

[Rickthebig]

I wish to get the current layer height while using variable layer height.
Since the layer_height variable only has the fixed nominal value, I need to use position[2] which is the value of the current height of the last two layers and build the difference. Then I only need to save that value in my own variable before it changes again. Something like:

global my_old_z=position[2]

Unfortunately, that does not work and I get an error, "my_old_z is not a variable". I tried all possible syntaxes.

Does Orca support custom variable assignments?
Prusa does, see:
https://github.com/prusa3d/PrusaSlicer/wiki/PrusaSlicer-Macro-Language