Summary:
-* Fixed_width Priority
+* **Fixed_width Priority**
- all operations (add, mul, sub, shift, div)
expected to work (caveat: layout() should check rounding)
-* Element-width Priority
+* **Element-width Priority**
- all operations expected to work (no caveats)
-* Fixed-and-Elwidth (no priority)
+* **Fixed-and-Elwidth (no priority)**
- multiply and left-shift always expected to work
- divide and right-shift expected to work (caveat: no bits
lost due to rounding)