I am part way through implementing a Rational math class to further my 
understanding of Ruby and had a couple of questions that I can't find 
the answer to in Pickaxe.
The first question regards creating a new instance of the class.  The 
initialize method expects two integer values.  While I have no problem 
making sure that they are integers I am not sure what to do if they are 
not.  Do I return nil or some other result?
The other question is how to override basic math operations like 
addition and multiplication.  I could implement them as x.add(y) but 
would prefer to just be able to enter x + y.  I think I have to create a 
new instance of the class for the result and return that but am not sure 
how to make it so that Ruby calls the right method when it sees x + y.
On a related note is there any good source for writing operations like 
the math and probably coerce?  If I can get the math working the 
comparable operations should be "trivial".