theorem Th16:
  i <= len G & j <= width G implies Int cell(G,i,j) is convex
proof
  assume that
A1: i <= len G and
A2: j <= width G;
  set P = Int cell(G,i,j);
A3: P = Int v_strip(G,i) /\ Int h_strip(G,j) by TOPS_1:17;
A4: Int v_strip(G,i) is convex by A1,Th12;
  Int h_strip(G,j) is convex by A2,Th11;
  hence thesis by A3,A4,Th5;
