% The generic transport kernel for a combinatorial geometry

% r: region number

t0 = physics(...); % Sample the distance to a phase-space changing event due to
                   % the physics of the situation

ri = r; % Current region number

ns = Ns(r); % Region r has ns bounding surfaces
for s = 1:ns

   [hit,t,rNext] = ...
      comgeom
      (  st(r,s), ... % The s'th boundy surf type in r
         sp(r,s), ... % Associated surface parameters (linked lists are best)
         x, u,    ... % The particle's position and direction
         io(r,s)  ... % In region r, the particle is either inside or outside s
      )
      
   if (hit & t < t0)) % A bounding surface is struck
      r = rRnext; % The next region number
      t0 = t;

      % Transport the particle
      tally('before transport', ri, ...)
      x = x + u*t0;
      tally('after transport', rNext, ...)

   else % A physics event occurs inside the current elemental subvolume

      % Transport the particle
      tally('before transport', ri, ...)
      x = x + u*t0;
      tally('after transport', ...)

      tally('before physics', ...)
      [...] = ApplyPhysics(...); % Scattering, absorption, creation ...
      tally('after physics', ...)

   end