let p, q be Element of LTLB_WFF ; :: thesis: for X being Subset of LTLB_WFF st p 'or' q in tau X holds
( p in tau X & q in tau X )
let X be Subset of LTLB_WFF; :: thesis: ( p 'or' q in tau X implies ( p in tau X & q in tau X ) )
assume p 'or' q in tau X ; :: thesis: ( p in tau X & q in tau X )
then ('not' p) '&&' ('not' q) in tau X by Th19;
then ( 'not' p in tau X & 'not' q in tau X ) by Th20;
hence ( p in tau X & q in tau X ) by Th19; :: thesis: verum