after much discussion about the implementation. Since it is not
exactly clear what the field lines do at corners, the implementation
depends largely on an unknown and presumably global constant
multiplier; this has provisionally been set to 1 but needs to be
determined empirically with a field equation solver. At least one
rational approximation suggests a value of pi/2 instead of 1. The
corner capacitance can be considered a refinement of existing
parasitic capacitance extraction, and since it takes a non-trivial
additional amount of computation, it is left as an option to
"extract" that can be turned on when a more accurate result is
preferred at the expense of a longer extraction time.
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