Using multiple times defines duplicate of `validate_state_transition`

[h4cc]

Fix would be to use this:

      events
      |> Enum.each(fn(event) ->
        unless event[:to] in states do
          raise "Target state :#{event[:to]} is not present in @states"
        end

        def unquote(event[:name])(model) do
          model
          |> Changeset.change(%{ unquote(column) => "#{unquote(event[:to])}" })
          |> unquote(event[:callback]).()
          |> unquote(:"validate_state_transition_#{column}")(unquote(event), model)
        end

        def unquote(:"can_#{event[:name]}?")(model) do
          :"#{Map.get(model, unquote(column))}" in unquote(event[:from])
        end
      end)

      defp unquote(:"validate_state_transition_#{column}")(changeset, event, model) do
        change = Map.get(model, unquote(column))

        if :"#{change}" in event[:from] do
          changeset
        else
          changeset
          |> Changeset.add_error(unquote(column),
            "You can't move state from :#{change} to :#{event[:to]}"
            )
        end
      end