reserve
  a for natural Number,
  k,l,m,n,k1,b,c,i for Nat,
  x,y,z,y1,y2 for object,
  X,Y for set,
  f,g for Function;
reserve p,q,r,s,t for FinSequence;
reserve D for set;
reserve a, b, c, d, e, f for object;

theorem
 dom <*x,y*> = { 1,2 }
 proof
  thus dom <*x,y*> = Seg len <*x,y*> by Def3
          .= {1,2} by Th2,Th44;
 end;
