reserve r,lambda for Real,
  i,j,n for Nat;
reserve p,p1,p2,q1,q2 for Point of TOP-REAL 2,
  P, P1 for Subset of TOP-REAL 2;
reserve T for TopSpace;

theorem Th21:
  for f being FinSequence of TOP-REAL n st 1 <= i & i+1 <= len f
  holds f/.i in LSeg(f,i) & f/.(i+1) in LSeg(f,i)
proof
  let f be FinSequence of TOP-REAL n;
  assume that
A1: 1 <= i and
A2: i+1 <= len f;
  LSeg(f,i) = LSeg(f/.i,f/.(i+1)) by A1,A2,Def3;
  hence thesis by RLTOPSP1:68;
end;
