irbで2回以上loadすると失敗する

rubyのirbでrubyのコードを2回以上loadすると失敗します。
なぜなのでしょうか?

pythonだと

import mymodule
import importlib

importlib.reload(mymodule) # 変更が反映される

とするとモジュールの変更が反映されますが

rubyだとどうするのでしょうか?

>> load "./big_step.rb" #2回目
TypeError: superclass mismatch for class Number

big_step.rb

class Number < Struct.new(:value)
    def to_s
        return value.to_s
    end

    def evaluate(environment)
        self
    end

    def inspect
        return "<<#{self}>>"
    end
end


class Multiply < Struct.new(:left, :right)
    def to_s
        return "#{left} * #{right}"
    end

    def inspect
        return "<<#{self}>>"
    end

    def evaluate(environment)
        Number.new(left.evaluate(environment).value * right.environment(environment).value)
    end
end

class Add < Struct.new(:left, :right)
    def to_s
        return "#{left} + #{right}"
    end

    def inspect
        return "<<#{self}>>"
    end

    def evaluate(environment)
        Number.new(left.evaluate(environment).value + right.evaluate(environment).value)
    end
end

class Boolean < Struct.new(:value)
    def to_s
         value.to_s
    end

    def inspect
        "<<#{self}>>"
    end

    def evaluate(environment)
        self
    end
end

class LessThan < Struct.new(:left, :right)
    def to_s
        "#{left} < #{right}"
    end

    def inspect
        "<<#{self}>>"
    end

end

class Variable < Struct.new(:name)
    def to_s
        name.to_s
    end

    def inspect
        "<<#{self}>>"
    end

    def evaluate(environment)
        environment[name]
    end
end

class DoNothing
    def to_s
        "do-noting"
    end

    def inspect
        "<<#{self}>>"
    end

    def ==(other_statement)
        other_statement.instance_of?(DoNothing)
    end
end


class Assign < Struct.new(:name, :expression)
    def to_s
        "#{name} = #{expression}"
    end

    def inspect
        "<<#{self}>>"
    end

    def evaluate(environment)
        environment.merge({name => expression.evaluate(environment)})
    end
end


class If < Struct.new(:condition, :true_exe, :false_exe)
    def to_s
        "if (#{condition}) { #{true_exe} } else { #{false_exe} }"
    end

    def inspect
        "<<#{self}>>"
    end

    def evaluate(environment)
        case condition.evaluate(environment)
        when Boolean.new(true)
            true_exe.evaluate(environment)
        when Boolean.new(false)
            false_exe.evaluate(environment)
        end
    end
end

class Sequence < Struct.new(:first, :second)
    def to_s
        "#{first}; #{second}"
    end

    def inspect
        "<<#{self}>>"
    end


    def evaluate(environment)
        second.evaluate(first.evaluate(environment))
    end
end


class While < Struct.new(:condition, :body)
    def to_s
        "while (#{condition}) { #{body} }"
    end

    def inspect
        "<<#{self}>>"
    end

    def evaluate(environment)
        case condition.evaluate(environment)
        when Boolean.new(true)
            evaluate(body.evaluate(environment))
        when Boolean.new(false)
            environment
        end
    end
end