function P=perm(n,m) 
% perm(n,m)  m permutations of n 
if min(n)<0 
   error('undefined for n < 0') 
end 
if min(m)<0 
   error('undefined for m < 0') 
end 
if max(m>n)==1 
   error('undefined for m > n') 
end 
P=fact(n)./fact(n-m);