theorem
  the_argument_of F 'or' G = 'not' F '&' 'not' G & the_antecedent_of F
  'or' G = 'not' F & the_consequent_of F 'or' G = G
proof
  thus the_argument_of F 'or' G = 'not' F '&' 'not' G by Th3;
  F 'or' G = 'not' F => G;
  hence thesis by Th6;
end;
