
theorem th4:
  for p,q be boolean object holds
    (p => q) => (('not' q) => 'not' p) = 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 (p => q) => (('not' q) => 'not' p) = TRUE by A1;
  end;
