theorem
 for t1,t2 being expression of C st t1,t2 are_unifiable
  holds t1,t2 are_weakly-unifiable
  proof
   let t1,t2 be expression of C;
   given f being valuation of C such that
A1: f unifies t1,t2;
   take g = C idval variables_in t2;
   thus variables_in t2 c= dom g by ABCMIZ_1:131;
   take f; thus f unifies t1,t2 at g by A1,ABCMIZ_1:137;
  end;
