'Adds a distance penalty of 1.0 for each number of difference
between `number1` and `number2`, or 0.0 when there is no
difference. Use this when there is no upper limit on the
difference between the two numbers.'

def add_number(self, key, number1, number2):

    diff = abs((number1 - number2))
    if diff:
        for i in range(diff):
            self.add(key, 1.0)
    else:
        self.add(key, 0.0)
