reserve p,q,r for FinSequence;
reserve u,v,x,y,y1,y2,z for object, A,D,X,Y for set;
reserve i,j,k,l,m,n for Nat;

theorem Th27:
  len p = len q iff dom p = dom q
proof
  dom p = Seg(len p) by FINSEQ_1:def 3;
  hence thesis by FINSEQ_1:def 3;
end;
