theorem Th1:
  c in seq(a,b) iff 1+a <= c & c <= b+a
proof
A1: c in { m where m is Element of NAT: 1+a <= m & m <= b+a }
    iff ex m being Element of NAT st c = m & 1+a <= m & m <= b+a;
  c is Element of NAT by ORDINAL1:def 12;
  hence thesis by A1;
end;
