def urbanmain() = {

  import M5a._

  assert(sread(Map(), 2) == 0)
  assert(sread(Map(2 -> 1), 2) == 1)
  assert(write(Map(), 1, 2) == Map(1 -> 2))
  assert(write(Map(1 -> 0), 1, 2) == Map(1 -> 2))
}