Parents should be frozen after creation.

Use a tuple to ensure that the parents list
can not be mutated after flow creation.

Change-Id: Ic2a0ed73a2a7211184deb92d57720ff417429dca
Provide the length of the flows.

Its useful to be able to check if a flow contains
anything by just inspecting the len() of the flow.
This can be used to avoid running a flow in the
first place if nothing is in said flow.

Change-Id: Ibfea7f10fc67d5dd3ddfa6f6fe6c08805562998a
Merge "Provide the length of the flows."