7/18/2016 Added "The Handle Theorem" to section 5.7.
9/ 2/2017 Fixed typo in exmple 3.3.2, "3+3" -> "3+2"
1/30/2022 Fixed typo in proof of thm 2.1.2, extra upper limit on
\bigcap_{j\in I}^k A_{j}
3/ 1/2022 Fixed error/typo in statement of theorem 2.1.2.
3/ 7/2022 Modified proof of theorm 4.5.4.
Fixed definition of SDR, set --> sequence
4/11/2022 Fixed the proof of Menger's Theorem.
4/13/2022 Minor fix in the proof of 5.7.13.