In NewLeaf you have two equivalent ways of setting any property. You can call leaf.Set with a short (case-independent) code for the property, or you can run a property-setting function directly. In this case you must get the function name exactly correct! See the Property Reference for details.

leaf.Set ('font' 'ti,12')('sp' 0 18)
leaf.Font 'ti,12' ª leaf.Spacing 0 18

The two lines above are exactly equivalent; note that at least two characters are required to identify the property. To query the current value, you should use leaf.Query with a vector of property names.

The properties are all held in leaf.‘proplist, so to see a quick display of the available names, just type this variable to the APL session.

Whichever method you use, you will find that whenever you set a property you will be returned the previous value. This can be used to reverse a setting easily.