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 Th20:
  i <= len G & j <= width G implies cell(G,i,j) is convex
proof
  assume i <= len G & j <= width G;
  then v_strip(G,i) is convex & h_strip(G,j) is convex by Th18,Th19;
  hence thesis by GOBOARD9:6;
end;
