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.