1 

     2 assert(sread(Map(), 2) == 0)

     3 assert(sread(Map(2 -> 1), 2) == 1)

     4 assert(write(Map(), 1, 2) == Map(1 -> 2))

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