void _testevent (PARTICLE* p){
   p.alpha -= time_step;
   if (p.alpha <= 0) p.lifespan = 0;
 //p.z -= time_step * 2; // wont work too  
}
...
   PARTICLE* p = ent_decal (you, bmMark, 7+random(3), random(360));
   p.material  = mat_model; // mat_particle, mat_sprites doesnt matter
   p.alpha     = 80;
   p.vel_z     =  3;
   p.gravity   =  3;
   p.event     = _testevent;
   set (p, MOVE | TRANSLUCENT);
   /*//and this wont do it also
   while (p && p.z > 0){
      p.z -= time_step * 2;
      wait (1);
   }*/
...