:: Boolean Properties of Sets - Requirements
:: by Library Committee
::
:: Received April 30, 2002
:: Copyright (c) 2002-2011 Association of Mizar Users

begin

theorem :: BOOLE:1

for X being set holds X \/ {} = X

proof end;

theorem :: BOOLE:2

for X being set holds X /\ {} = {}

proof end;

theorem :: BOOLE:3

for X being set holds X \ {} = X

proof end;

theorem :: BOOLE:4

for X being set holds {} \ X = {}

proof end;

theorem :: BOOLE:5

for X being set holds X \+\ {} = X

proof end;

Lm1: for X being set st X is empty holds
X = {}

proof end;

theorem :: BOOLE:6

for X being set st X is empty holds
X = {} by Lm1;

theorem :: BOOLE:7

for x, X being set st x in X holds
not X is empty by XBOOLE_0:def 1;

theorem :: BOOLE:8

for X, Y being set st X is empty & X <> Y holds
not Y is empty

proof end;