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 Th51:
  len p = k + l & q = p | Seg k implies len q = k
proof
  assume that
A1: len p = k + l and
A2: q = p | Seg k;
  k <= len p by A1,NAT_1:12;
  hence thesis by A2,FINSEQ_1:17;
end;
