theorem Th19:
  j <= width G implies h_strip(G,j) is convex
proof
  assume
A1: j<= width G;
  set P = h_strip(G,j);
  let w1,w2 be Point of TOP-REAL 2 such that
A2: w1 in P & w2 in P;
  w1`2 <= w2`2 or w2`2 <= w1`2;
  hence thesis by A1,A2,Lm4;
end;
