Simplification does not preserve 'circuit-like-ness'

[obtext]

This may be related to #10; at least the symptom sounds the same.

Running the sequence of commands in the "Getting Started" document leads (usually) to an error after running Circuit.from_graph after clifford_simp, with the message "Graph doesn't seem circuit like: multiple parents".

A simple example that demonstrates the issue is:

def bad_graph():
    g = zx.Graph()
    g.add_vertices(8)
    g.set_type(2,1)
    g.set_type(3,1)
    g.set_type(4,2)
    g.set_type(5,1)
    g.set_position(0,0,0)
    g.set_position(1,1,0)
    g.set_position(2,0,1)
    g.set_position(3,1,1)
    g.set_position(4,0,2)
    g.set_position(5,1,2)
    g.set_position(6,0,3)
    g.set_position(7,1,3)
    g.add_edges([(0,2),(2,4),(4,6),(1,3),(3,5),(5,7),(4,5)])
    g.inputs.extend([0,1])
    g.outputs.extend([6,7])
    return g

g = bad_graph()
zx.simplify.clifford_simp(g)
Circuit.from_graph(g)

In fact, just doing spider_simp (the first step in clifford_simp) is enough to cause the error.

The problem is that the two green spiders on qubit 1 get merged to a position to the left of the (previously) red spider to which they remain connected.

I am not sure what the actual bug is or even if this is supposed to work -- but in any case the example code is not working.

[jvdwetering]

Hi, Thanks for the report. Circuit.from_graph does nothing fancy and hence only works if the ZX diagram already looks like a circuit. The line you are referring to in the getting started document: c = zx.Circuit.from_graph(g) is a typo and should have read:
c = zx.Circuit.from_graph(c)
which would have made sense in a previous version of PyZX where the extraction returned a circuit-like graph instead of a Circuit. I've updated the getting started document to reflect the change in PyZX.

[obtext]

OK, thanks!