reserve i for Integer,
  a, b, r, s for Real;

theorem Th31:
  a <= b & r <= s implies |[a,r]| in closed_inside_of_rectangle(a, b,r,s)
proof
  set o = |[a,r]|;
A1: closed_inside_of_rectangle(a,b,r,s) = {p where p is Point of TOP-REAL 2:
a <= p`1 & p`1 <= b & r <= p`2 & p`2 <= s} & o`1 = a by JGRAPH_6:def 2;
A2: o`2 = r;
  assume a <= b & r <= s;
  hence thesis by A1,A2;
end;
