I’m not sure about this but I suspect that it doesn’t involve parsing. It’s likely that the metapost expects a parameter or parameters to be passed to it which it then assigns to the variable Q in its own procedure. It won’t care what the calling program calls it. I think the arrowhead is probably defined within an arrow class (arrow.h perhaps).

On 30 Mar 2018, at 18:00, therion-request@speleo.sk wrote:

I notice line arrow takes the special argument -head [both | none | begin].
Somewhere between the th2 file and the metapost arrow definition, the
argument -head gets changed to the variable Q that is parsed to the metapost
code, that controls which ends of the arrow line get arrow heads. I have
copied the code below. Whereabouts is the code that tells Therion that
-head is parsed to the metapost definition as Q?

Dave Clucas
dave.clucas@icloud.com

Exploring the World - One cave at a time