ZHANG, Ruda January 18, 2020:  Convert math-text words to text only when the math symbols are ASCII only. [e4c945c]
ZHANG, Ruda December 30, 2018:  Use dollar style math markup. [1b7036c]
ZHANG, Ruda March 23, 2018:  Global remove trailing whitespaces [2b7ff90]
ZHANG, Ruda October 23, 2017:  More examples of variational principles [0fa39e9]
ZHANG, Ruda October 23, 2017:  digitize variational principles [bad00f3]