theorem Th7:
  seq(a,b) c= Seg (a+b)
proof
  let x be object;
A1: 1 <= 1+a by NAT_1:11;
  assume
A2: x in seq(a,b);
  then reconsider x as Element of NAT;
  1+a <= x by A2,Th1;
  then
A3: 1 <= x by A1,XXREAL_0:2;
  x <= b+a by A2,Th1;
  hence thesis by A3,FINSEQ_1:1;
end;
