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 '<' b & c '<' d implies a 'or' c '<' b 'or' d
proof
  assume a '<' b & c '<' d;
  then a 'imp' b = I_el(Y) & c 'imp' d = I_el(Y) by BVFUNC_1:16;
  then (a 'or' c) 'imp' (b 'or' d) = I_el(Y) by Th22;
  hence thesis by BVFUNC_1:16;
end;
