def add_variables(vars)
      # Store all the variables in the model
      # Most of the work will be done when we add the constraints
      vars.each do |var|
        var.model = self
        var.index = @variables.count
        @variables << var
      end
    end
def add_variables(vars)
      # Store all the variables in the model
      # Most of the work will be done when we add the constraints
      vars.each do |var|
        var.model = self
        var.index = @variables.count
        @variables << var
      end
    end
Add multiple variables to the model simultaneously