System A (textbook trap) = {1, 3, 4}
  greedy FAILS.  smallest counterexample: amount = 6
    greedy : 4 + 1 + 1   = 3 coins
    optimal: 3 + 3   = 2 coins
  greedy is wrong for 49 of the 200 amounts tested

System B (no nickel) = {1, 10, 25}
  greedy FAILS.  smallest counterexample: amount = 30
    greedy : 25 + 1 + 1 + 1 + 1 + 1   = 6 coins
    optimal: 10 + 10 + 10   = 3 coins
  greedy is wrong for 70 of the 200 amounts tested

System C (US coins) = {1, 5, 10, 25}
  greedy is optimal for every amount from 1 to 200

System D (rupiah, x1000) = {1, 2, 5, 10, 20, 50, 100}
  greedy is optimal for every amount from 1 to 200

System E (powers of 2) = {1, 2, 4, 8, 16, 32, 64}
  greedy is optimal for every amount from 1 to 200

System F (rupiah minus 2) = {1, 5, 10, 20, 50, 100}
  greedy is optimal for every amount from 1 to 200
