function H = msgpass( Xpot, Rpot, Ypot, varsizes )
%
% function H = msgpass( X, R, Y, varDomSize )
%   computes a single message pass from X to Y
%   tree structure: (X)--[R]--(Y)
%  
global varDomSize;
varDomSize = varsizes;

% projection
oldRpot = Rpot;
biggerset = Xpot.vars;
sumoutset = setdiff(Xpot.vars,Rpot.vars);
newpot = Xpot;
for i=1:length(sumoutset)
  newpot = mp_addout( newpot, sumoutset(i) );
end 
Rpot = newpot;
% absorption
if any(oldRpot.vals(:)) == 0, % if potential = 0 then set dividend to 0
  tmppot = mk_init_mp0( Rpot.vars, varsizes );
else
  tmppot = mp_div( Rpot, oldRpot );
end;
  Ypot = mp_mult( Ypot, tmppot );
H{1} = Xpot;
H{2} = Rpot;
H{3} = Ypot;