
[;1m  get_keys(Val)[0m

  Returns a list of keys that are associated with the value [;;4mVal[0m in
  the process dictionary. The items in the returned list can be in
  any order.

  For example:

    > put(mary, {1, 2}),
    put(had, {1, 2}),
    put(a, {1, 2}),
    put(little, {1, 2}),
    put(dog, {1, 3}),
    put(lamb, {1, 2}),
    get_keys({1, 2}).
    [mary,had,a,little,lamb]
