-- The module attribute ``abc_box_id`` specifies a positive integer linking a
- blackbox or whitebox definition to a corresponding entry in a `abc9`
- box-file.
-
-- The port attribute ``abc_scc_break`` indicates a module input port that will
- be treated as a primary output during `abc9` techmapping. Doing so eliminates
- the possibility of a strongly-connected component (i.e. a combinatorial loop)
- existing. Typically, this is specified for sequential inputs on otherwise
- combinatorial boxes -- for example, applying ``abc_scc_break`` onto the `D`
- port of a LUTRAM cell prevents `abc9` from interpreting any `Q` -> `D` paths
- as a combinatorial loop.
-
-- The port attribute ``abc_carry`` marks the carry-in (if an input port) and
- carry-out (if output port) ports of a box. This information is necessary for
- `abc9` to preserve the integrity of carry-chains. Specifying this attribute
- onto a bus port will affect only its most significant bit.
-