
theorem th3a:
  for p,q being boolean object holds
   'not' (p 'or' q) <=> ('not' p) '&' ('not' q) = TRUE
  proof
   let p,q be boolean object;
A1: p = TRUE or p = FALSE by XBOOLEAN:def 3;
   q = TRUE or q = FALSE by XBOOLEAN:def 3;
   hence 'not' (p 'or' q) <=> ('not' p) '&' ('not' q) = TRUE by A1;
  end;
