theorem Th10:
  Evaluate(H1 'or' H2,Kai) = Evaluate(H1,Kai) 'or' Evaluate(H2,Kai )
proof
  Evaluate(H1 'or' H2,Kai) = 'not' Evaluate('not' H1 '&' 'not' H2,Kai) by Th5
    .= 'not' (Evaluate('not' H1,Kai) '&' Evaluate('not' H2,Kai)) by Th6
    .= 'not' (('not' Evaluate(H1,Kai)) '&' Evaluate('not' H2,Kai)) by Th5;
  hence thesis by Th5;
end;
