I have the following code
and the result is two arrows from a to b, I want to put \vdots between the two arrows, exactly in the middle (to indicate there are many arrows from a to b). Could anyone help with this?
Other methods not using
\xymatrix or \rtwocell producing horizontal parallel arrows with 3 vertical dots between them is also welcome and is fine for me. What I want is roughly as follows (but I want it to be more symmetric):