"Xmax is used but not defined", but it appears defined in the config

[kyle-kingsbury]

If you go to IPAddress/config there is clearly an xmax variable that can be adjusted using configuration->Machine->Machine Envelope-> X Max. However, if you try to access it in an opensbp file such as:

so,1,1
mz, 0
mx, $XMAX

Fabmo presents an error message that the variable is undefined.