function p = poisson(N,lambda)
%p = poisson(N,lambda)

if length(N)==1,
   p = exp(-lambda)*lambda^N / prod(1:N);
else, %% If the input is a vector, give back the Poisson evaluated
   	%% in as many samples.
   N = 1:length(N);
   p = exp(-lambda)*lambda.^[0 N] ./ cumprod([1 N]);
end;