theorem Th24:
  cell(G,0,0) = { |[r,s]| : r <= G*(1,1)`1 & s <= G*(1,1)`2 }
proof
A1: cell(G,0,0) = v_strip(G,0) /\ h_strip(G,0) by GOBOARD5:def 3;
A2: h_strip(G,0) = { |[r,s]| : s <= G*(1,1)`2 } by Th21;
A3: v_strip(G,0) = { |[r,s]| : r <= G*(1,1)`1 } by Th18;
  thus cell(G,0,0) c= { |[r,s]| : r <= G*(1,1)`1 & s <= G*(1,1)`2 }
  proof
    let x be object;
    assume
A4: x in cell(G,0,0);
    then x in v_strip(G,0) by A1,XBOOLE_0:def 4;
    then consider r1,s1 such that
A5: x = |[r1,s1]| and
A6: r1 <= G*(1,1)`1 by A3;
    x in h_strip(G,0) by A1,A4,XBOOLE_0:def 4;
    then consider r2,s2 such that
A7: x = |[r2,s2]| and
A8: s2 <= G*(1,1)`2 by A2;
    s1 = s2 by A5,A7,SPPOL_2:1;
    hence thesis by A5,A6,A8;
  end;
  let x be object;
  assume x in { |[r,s]| : r <= G*(1,1)`1 & s <= G*(1,1)`2 };
  then
A9: ex r,s st x = |[r,s]| & r <= G*(1,1)`1 & s <= G*(1,1)`2;
  then
A10: x in h_strip(G,0) by A2;
  x in v_strip(G,0) by A3,A9;
  hence thesis by A1,A10,XBOOLE_0:def 4;
end;
