My idea was to use the brackets from musical notation as inspiration;
if you know some principles of typography, it is rather straight-forward
to come up with a pleasing design of such a bracket, using a
cascade of golden ratio relationships.
BUT ... all of this is geometry, and translating that into a symbolic
or numerical calculation is excessively complicated. Thus I looked
for ways to use some geometry or CAD software to build such a construction.
The geometry software I tired was woefully inadequate for this task.
Using the Constraint system in FreeCAD, building the construction went
smooth and straight forward, but then I was unable to export that drawing
in a way indicating the construction clearly.
So in the end, I'll have to hand-pick the resulting numerical coordinates
from the FreeCAD XML document and integrate them directly into Cairo
drawing code...