Covered in [[biginteger/analysis]] the summary is that standard `adde`
is sufficient for SVP64 Vectorisation of big-integer addition (and subfe
-for subtraction) but that big-integer multiply and divide require an
-extra 3-in 2-out instruction, similar to Intel's `mulx`, to be efficient.
+for subtraction) but that big-integer shift, multiply and divide require an
+extra 3-in 2-out instructions, similar to Intel's `mulx` and
+`idiv`, to be efficient.
The same instruction (`maddedu`) is used for both because 'maddedu''s primary
purpose is to perform a fused 64-bit scalar multiply with a large vector,
where that result is Big-Added for Big-Multiply, but Big-Subtracted for