theorem
  the_left_argument_of (p 'or' q) = p & the_right_argument_of (p 'or' q) = q
proof
  p 'or' q is disjunctive;
  then
  p 'or' q = (the_left_argument_of (p 'or' q)) 'or' (the_right_argument_of
  (p 'or' q)) by ZF_LANG:41;
  hence thesis by ZF_LANG:31;
end;
