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 Th18:
  i <= len G implies v_strip(G,i) is convex
proof
  assume
A1: i<= len G;
  let w1,w2 be Point of TOP-REAL 2;
  set P = v_strip(G,i);
A2: w1`1 <= w2`1 or w2`1 <= w1`1;
  assume w1 in P & w2 in P;
  hence thesis by A1,A2,Lm3;
end;
