Python co-routine to store a single value

siso.py

from collections import namedtuple

Arrow = namedtuple('Arrow', 'tail head')

# Single input single output
def siso():
    def inner():
        x = yield
        yield
        yield x
    g = inner()
    next(g)
    return g

# Single operation of simplifying the state by removing replacing arrows with
# their heads where possible.
def search_step(state, did_simplify):
    simplified = False
    for element in state:
        if isinstance(element, Arrow) and element.tail in state:
            yield element.head
            simplified = True
        else:
            yield element
    did_simplify.send(simplified)

# Search until state is simplified as much as possible. Detect whether to
# continue searching using the siso.
def search(state):
    while True:
        did_simplify = siso()
        state = set(search_step(state, did_simplify))
        if not next(did_simplify):
            break
    return state

state = {
    1, 2, 3, 4,
    Arrow(1, 5),
    Arrow(2, 6),
    Arrow(6, 7),
    Arrow(7, Arrow(3, 8)),
}

print(list(search(state)))
"""
[1, 2, 3, 4, 5, 6, 7, 8]
"""