theorem Th27:
  s,t1, V-separate s,t2
proof
  let A be Subset of TOP-REAL n such that
A1: A is_an_arc_of s,t1 and
  A c= V;
A2: s in {s,t2} by TARSKI:def 2;
  s in A by A1,TOPREAL1:1;
  hence thesis by A2,XBOOLE_0:3;
end;
