Exercise 2.15: Eva Lu Ator, another user, has
also noticed the different intervals computed by different but algebraically
equivalent expressions. She says that a formula to compute with intervals using
Alyssa’s system will produce tighter error bounds if it can be written in such
a form that no variable that represents an uncertain number is repeated.  Thus,
she says, par2 is a “better” program for parallel resistances than
par1.  Is she right?  Why?