class Material:
    pass


class Vacuum(Material):
    def __str__(self):
        return 'vacuum'


class Aluminum(Material):
    def __str__(self):
        return 'aluminum'


class Graphite(Material):
    def __str__(self):
        return 'graphite'


class Beryllium(Material):
    def __str__(self):
        return 'beryllium'

allowed_materials = ['vacuum', 'aluminum', 'graphite',]