function retconc = CombinatoricConcentration(conc, stoc)
% function to compute the combinatoric term in Gillespie reaction
% propensity
if stoc == 0
    retconc = 1;
else if conc <= 0
    retconc = 0;
else
  retconc = 1;
  for i=1:stoc
    retconc = retconc*(conc-(i-1));
  end
  retconc = retconc/factorial(stoc);
end
end