reserve Y for non empty set;
reserve Y for non empty set;
reserve Y for non empty set;
reserve Y for non empty set,
  a,b,c,d,e,f,g for Function of Y,BOOLEAN;

theorem
  (a 'imp' b) '&' (a 'imp' c) '<' a 'imp' (b 'or' c)
proof
  (a 'imp' b) '&' (a 'imp' c) '<' (a 'or' a) 'imp' (b 'or' c) by Th21;
  hence thesis;
end;
