POWERS MODULO N

modppower(b,e,m) computes b^e mod m using the 
method of repeated squaring.

Example: 

   >>> modpower(2,100,101)
   1

Check: Fermat's little theorem.

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def modpower(b,e,m):
  """modpower(b,e,m) = b^e mod m"""
  P = 1
  S = b
  while e > 0:
    r = e%2
    e = e/2
    if r == 1:
	  P = P*S % m
    S = S*S % m
  return