Nodes of different colours represent the following:
Solid arrows point from a procedure to one which it calls. Dashed
arrows point from an interface to procedures which implement that interface.
This could include the module procedures in a generic interface or the
implementation in a submodule of an interface in a parent module.
Where possible, edges connecting nodes are
given different colours to make them easier to distinguish in
large graphs.
public module subroutine open_potential_history_writer(app, resumed, potential_history_opened, potential_history_unit)