code 41252;
real procedure POISSONPROB(X, MU);
value X, MU; real X, MU;
POISSONPROB:= if MU ≤ 0
              then STATAL3 ERROR(“POISSONPROB”, 2, MU)
              else if X < 0 ∨ X > ENTIER(X) then 0
              else EXP(-MU + X × LN(MU) - LOGGAMMA(X+1));
eop