another_calc.rb

module AnotherCalc
  def perform(product)
    case product
    when "firstcase"
      do_something
    when "secondcase"
      do_something_else
    when "thirdcase"
      do_something_more
    when "fourthcase"
      do_something_even_more_different
    end
  end
end

biz_calc.rb

module BizCalc

  def rule_1(product)
  end
  
  def rule_2(product)
  end
  
  def rule_3(product)
    result = some_shared_function(x)
  end
  
  def rule_4(product)
    result = some_shared_function(x)
  end
  
  def some_shared_function(vals)
  end
end