Executing an empty Concurrence container hangs forever

[okalachev]

Let's say one have some code like:

cc = Concurrence(['foo'], default_outcome='foo')

with cc:
    if add_bar:
        Concurrence.add('bar', Bar())
    if add_baz:
        Concurrence.add('baz', Baz())
cc = execute()

If both conditions are negative, the execute method will hang forever. I guess, this is pretty unexpected and confusing behaviour, and may cause strange bugs.

I propose to raise an exception in such situations.

[Timple]

Or immediately return the default outcome could also be an option.

[130s]

Thank you for contribution.

I have not tested nor thoroughly reviewed but brief look the goal you describe seems achieved.

Merging and will depend on the community to further test.