fun float tritest_b( vec o, vec d, vec p0, vec p1, vec p2 ) {
  vec normal = ( vec edge0 = p1 - p0 ) x ( vec edge1 = p0 - p2 );
  float rcp = 1.0 / normal . d;
  float toverd = normal . ( vec edge2 = p0 - o ) * rcp;
  if ( toverd < epsilon )
    return nohit;
  float uoverd = ( vec interm = edge2 x d ) . edge1 * rcp;
  if ( uoverd < 0.0 )
    return nohit;
  float voverd = interm . edge0 * rcp;
  if ( uoverd + voverd > 1.0 || voverd < 0.0 )
    return nohit;
  return toverd;
}