Representation of arguments to a module instance (for ports, not parameters).
This is a tagged union type, introduced by deftagsum.
There are two kinds of argument lists for the ports of module instantiations, which we call plain and named arguments.
modname instname ( 1, 2, 3 ); <-- "plain" arguments modname instname ( .a(1), .b(2), .c(3) ); <-- "named" arguments
A