reserve i,j,k,n for Nat,
  X,Y,a,b,c,x for set,
  r,s for Real;
reserve f,g for FinSequence of TOP-REAL 2;
reserve f for non constant standard special_circular_sequence,
  p,p1,p2,q for Point of TOP-REAL 2;
reserve G for Go-board;

theorem Th26:
  for p,p1,p2 st not p in LSeg(p1,p2) & p1`1 = p2`1 & p2`1 = p`1
  holds p1 in LSeg(p,p2) or p2 in LSeg(p,p1)
proof
  let p,p1,p2 such that
A1: not p in LSeg(p1,p2) and
A2: p1`1 = p2`1 & p2`1 = p`1;
  per cases;
  suppose
A3: p1`2 <= p2`2;
    now
      per cases by A1,A2,GOBOARD7:7;
      suppose
        p`2<p1`2;
        hence thesis by A2,A3,GOBOARD7:7;
      end;
      suppose
        p2`2<p`2;
        hence thesis by A2,A3,GOBOARD7:7;
      end;
    end;
    hence thesis;
  end;
  suppose
A4: p2`2 <= p1`2;
    now
      per cases by A1,A2,GOBOARD7:7;
      suppose
        p`2<p2`2;
        hence thesis by A2,A4,GOBOARD7:7;
      end;
      suppose
        p1`2<p`2;
        hence thesis by A2,A4,GOBOARD7:7;
      end;
    end;
    hence thesis;
  end;
end;
