reserve p,p1,p2,q for Point of TOP-REAL 2,
  f,f1,f2,g,g1,g2 for FinSequence of TOP-REAL 2,
  r,s for Real,

  n,m,i,j,k for Nat,
  G for Go-board,
  x for set;

theorem Th9:
  f <> {} implies X_axis(f) <> {}
proof
A1: len X_axis(f) = len f by GOBOARD1:def 1;
  assume f <> {} & X_axis(f) = {};
  hence contradiction by A1;
end;
