theorem Th20:
  ((a 'imp' c) '&' (b 'imp' c)) '<' (a 'or' b) 'imp' c
proof
  ((a 'imp' c) '&' (b 'imp' c)) 'imp' ((a 'or' b) 'imp' c) = I_el Y by
Th9;
  hence thesis by BVFUNC_1:16;
end;
