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 Th24:
  for p,q,r,s being Point of TOP-REAL 2 st LSeg(p,q) is vertical &
  LSeg(r,s) is vertical & LSeg(p,q) meets LSeg(r,s) holds p`1 = r`1
proof
  let p,q,r,s be Point of TOP-REAL 2 such that
A1: LSeg(p,q) is vertical and
A2: LSeg(r,s) is vertical;
  assume LSeg(p,q) meets LSeg(r,s);
  then LSeg(p,q) /\ LSeg(r,s) <> {};
  then consider x being Point of TOP-REAL 2 such that
A3: x in LSeg(p,q) /\ LSeg(r,s) by SUBSET_1:4;
A4: x in LSeg(r,s) by A3,XBOOLE_0:def 4;
  x in LSeg(p,q) by A3,XBOOLE_0:def 4;
  hence p`1 = x`1 by A1,SPPOL_1:41
    .= r`1 by A2,A4,SPPOL_1:41;
end;
