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;
reserve Y for non empty set;

theorem
  for a,b,c being Function of Y,BOOLEAN holds (c 'imp' a)=I_el(Y)
  & (c 'imp' b)=I_el(Y) implies c 'imp' (a 'or' b)=I_el(Y)
proof
  let a,b,c be Function of Y,BOOLEAN;
  assume
A1: (c 'imp' a)=I_el(Y) & (c 'imp' b)=I_el(Y);
  c 'imp' (a 'or' b) =(c 'imp' a) 'or' (c 'imp' b) by Th73
    .=I_el(Y) by A1;
  hence thesis;
end;
