This method is called when a `Rational`

instance appears on the left side of the “`-`

” operator. The right-hand operand must
be a `Rational`

instance as well. See Section 3.5, “`Rational.__add__()`

: Implement the
addition (`+`

) operator” for the algebra of this
operation.

rational.py

def __sub__ ( self, other ): """Return self minus other. """ return Rational ( self.n * other.d - other.n * self.d, self.d * other.d )