Yes, indeed 'canonical' can be justified to take an integer, if we interpret the spec as:

'canonical' takes an operand and returns the preferred _decimal_
encoding of that operand.

But then 'is_canonical' should return false for an integer, and
this would create another special case: Accept an operand and
return something meaningful _without_ converting it first.

I think this is why I have problems with those two. 'number_class'
is less of a problem.