theorem Th52:
  a < b & c < d implies
  Fr closed_inside_of_rectangle(a,b,c,d) = rectangle(a,b,c,d)
proof
  assume that
A1: a < b and
A2: c < d;
  set P = closed_inside_of_rectangle(a,b,c,d);
  thus Fr P = P \ Int P by TOPS_1:43
    .= P \ inside_of_rectangle(a,b,c,d) by A1,A2,Th50
    .= rectangle(a,b,c,d) by A1,A2,Th51;
end;
