A sphere cut based on the distance towards two arcs.
float dist_0 = xyzdist(1, '0', v@P, int(0), vector(0.0));
float dist_1 = xyzdist(1, '1', v@P, int(0), vector(0.0));
f@dist = abs(dist_1 - dist_0);
The gap is defined by subtracting a custom width from the subtraction of both distances.
float gap = chf('gap');
v@P.y = f@dist - gap;