TO DO:
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Eliminate unnecessary stuff from router module, particularly user channel
    definition using labels.
Via minimization.

TEST:
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Temporary hack: mechanism to use pre-routed stems provided by AMCC
    (possibly an "alias" command to the router that sets up a table
     mapping terminal names in the netlist to new names).
Check for blockages in the channel; eliminate blocked layers from the
    list of layers we try to connect to during stem generation.
Stem generation: be generous in allowing stems to use above grid
    lines as well as below.