Derive addition, multiplication, and exponentiation from from `Integer#next`

math.rb

def add(number1, number2)
  number2.times.reduce(number1) { |total| total.next }
end                                                    

add(2,3)
# => 5

def subtract(number1, number2)
  number2.times.reduce(number1) { |total| total.pred }
end

subtract(5, 3)
# => 2
                                                               
def multiply(number1, number2)
  number1.times.reduce(0) { |total| add(total, number2) }
end                                                   

multiply(2, 3)
# => 6

def exponent(base, power)
  power.times.reduce(1) { |total| multiply(total, base) }
end

exponent(2, 3)                                        
#=> 8 

See also https://gist.github.com/JoelQ/b5356aaa80e2e032c6cabfe79d5d0da5