**Description:** The null set is a subset of any class. Part of Exercise 1 of
TakeutiZaring p. 22. (Contributed by NM, 21-Jun-1993)

Ref | Expression | ||
---|---|---|---|

Assertion | 0ss | $${\u22a2}\varnothing \subseteq {A}$$ |

Step | Hyp | Ref | Expression |
---|---|---|---|

1 | noel | $${\u22a2}\neg {x}\in \varnothing $$ | |

2 | 1 | pm2.21i | $${\u22a2}{x}\in \varnothing \to {x}\in {A}$$ |

3 | 2 | ssriv | $${\u22a2}\varnothing \subseteq {A}$$ |