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 Th33:
  a '<' b & c '<' d implies a '&' c '<' b '&' 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 '&' c) 'imp' (b '&' d) = I_el Y by tt;
  hence thesis by BVFUNC_1:16;
end;
