-costly 4-reg (VA-Form) a 2-bit mode `sm` gives four
-potential overwrite and zero-source options instead.
+costly 4-reg (VA-Form) and to allow Vectorised versions
+to use EXTRA3 a 2-bit mode `sm` gives four
+potential overwrite and zero-source options instead
+(a 4-reg VA-Form Vector variant would use EXTRA2,
+limiting the vector offset range and preventing its
+use in biginteger vector-shift operations).