theorem
 for t1,t2,t being expression of C st t is_a_unification_of t1,t2
  holds t is_a_unification_of t2,t1
  proof
   let t1,t2,t be expression of C;
   given f being valuation of C such that
A1: f unifies t1,t2 & t = t1 at f;
   take f;
   thus f unifies t2,t1 & t = t2 at f by A1;
  end;
