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A 2vecx is a 4vec that is either a 2vec or is all Xes.
In the Verilog semantics, many of the more interesting vector operations (e.g., addition, multiplication, etc.) have a special X propagation behavior where, if any bit of any input is X or Z, then the entire result is X.
The 2vecx'es are a subset of the 4vecs that capture this idea. That is, the result of an operation like addition is surely a 2vecx.