In Ruby, it seems that a lot of coerce() help can be done by

def coerce(something)
  [self, something]
end

that's is, when

3 + rational

is needed, Fixnum 3 doesn't know how to handle adding a Rational, so it asks Rational#coerce for help by calling rational.coerce(3), and this coerce instance method will tell the caller:

# I know how to handle rational + something, so I will return you the following:
[self, something]
# so that now you can invoke + on me, and I will deal with Fixnum to get an answer

So what if most operators can use this method, but not when it is (a - b) != (b - a) situation? Can coerce() know which operator it is, and just handle those special cases, while just using the simple [self, something] to handle all the other cases where (a op b) == (b op a) ? (op is the operator).