Po:=proc(k,a) local n:
if evalb((whattype(k) = symbol) and (whattype(a) = symbol)) then
   Physics[Assume](k >= 0, a, 'real'):
fi:
limit(n!/(k!*(n-k)!)*(a^k)/(n^k)*(1-a/n)^(n-k),n=infinity):
end: