def _find_install_subgraphs(graph):
    """In the install graph, find subgraphs that install a node instance.

    Make a dict of {instance id: subgraph}, based on the subgraph name.
    """
    install_subgraphs = {}
    for task in graph.tasks:
        if task.is_subgraph and task.name.startswith('install_'):
            instance_name = task.name[len('install_'):]
            install_subgraphs[instance_name] = task
    return install_subgraphs