theorem Th4:
  dom <*a,b,c*> = {1,2,3}
  proof
    thus dom <*a,b,c*> = Seg len <*a,b,c*> by FINSEQ_1:def 3
    .= {1,2,3} by FINSEQ_1:45,FINSEQ_3:1;
  end;
