# This function computes all permutations
def permute (a, lo):
  hi = len(a)
  if (lo == hi):
    print (a)
  else:
    for i in range (lo, hi):
      a[lo], a[i] = a[i], a[lo]
      permute (a, lo + 1)
      a[lo], a[i] = a[i], a[lo]

def main ():
  a = ['A', 'B', 'C', 'D']
  permute (a, 0)

main()

# This function computes all subsets of a list
def subsets (a, b, lo):
  hi = len(a)
  if (lo == hi):
    print (b)
    return
  else:
    c = b[:]
    b.append (a[lo])
    subsets (a, c, lo + 1)
    subsets (a, b, lo + 1)

def main():
  a = [15, 9, 30, 21, 19, 3, 12, 6, 25, 27]
  b = []
  subsets (a, b, 0)

main()

# This function computes all combinations of a given size
def combine (a, b, lo, size):
  hi = len(a)
  if (lo == hi):
    if (len(b) == size):
      print (b)
    return
  
  if (len(b) == size):
    print (b)
  else:
    c = b[:]
    b.append(a[lo])
    combine (a, c, lo + 1, size)
    combine (a, b, lo + 1, size)

def main():
  a = ['A', 'B', 'C', 'D', 'E', 'F']
  b = []
  combine (a, b, 0, 3)

main()