theorem Th4:
  the_left_argument_of (p '&' q) = p & the_right_argument_of (p '&' q) = q
proof
  p '&' q is conjunctive;
  then
  p '&' q = (the_left_argument_of (p '&' q)) '&' (the_right_argument_of (p
  '&' q)) by ZF_LANG:40;
  hence thesis by ZF_LANG:30;
end;
