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 Th28:
  len p <= len q iff dom p c= dom q
proof
A1: dom q = Seg(len q) by FINSEQ_1:def 3;
  dom p = Seg(len p) by FINSEQ_1:def 3;
  hence thesis by A1,FINSEQ_1:5;
end;
