Stacking BRAY-transparent elements on top of non-transparent elements makes ARAY ignore the non-trasparent particle stacked under transparent one when it (the ARAY) is calculating the distance to the first non-transparent particle; particles in stack (except the particle at the top of the stack) are invisible to particles outside the stack.  But, somehow, particles stacked under FILT aren't invisible to particles outside the stack.  BRAY doesn't pass through FILT with a non-transparent particle stacked under it and while experimenting with CRAY(SPRK), I obseved that it deleted the particle stacked under FILT; I was expecting the CRAY to ignore the particle stacked under and had to use DMND at the top of the stack instead.  How is it that particles stacked under FILT are still visible to particles outside the stack?