let p, q be boolean object ; :: thesis: ('not' p) '&' (p => q) = ('not' p) 'or' (('not' p) '&' q)
( p = FALSE or p = TRUE ) by Def3;
hence ('not' p) '&' (p => q) = ('not' p) 'or' (('not' p) '&' q) ; :: thesis: verum