Add a new node and its predecessors to the graph.
Both the *node* and all elements in *predecessors* must be hashable.
If called multiple times with the same node argument, the set of dependencies
will be the union of all dependencies passed in.
It is possible to add a node with no dependencies (*predecessors* is not provided)
as well as provide a dependency twice. If a node that has not been provided before
is included among *predecessors* it will be automatically added to the graph with
no predecessors of its own.
Raises ValueError if called after "prepare".