reserve k,n for Nat,
  x,y,z,y1,y2 for object,X,Y for set,
  f,g for Function;
reserve p,q,r,s,t for XFinSequence;
reserve D for set;

theorem Th51: :: FINSEQ_1:12
  n <= len p implies len(p|n) = n
 proof
  assume n <= len p;
   then Segm n c= Segm len p by NAT_1:39;
   hence thesis by RELAT_1:62;
 end;
