new file:   doc/Farey sequence.pdf
	modified:   mytools.py   add function phi
	modified:   p69.py
	new file:   p70.py
	new file:   p71.py        using Farey sequence to solve it
	new file:   doc/Pythagorean triple.pdf
	new file:   p72.py
	new file:   p73.py
	new file:   p74.py