Chapter 2.4.3: math_atan2 should be used for two-argument arctangent

[PossibilityZero]

Code snippets in Chapter 2.4.3 call the math_atan function with two arguments (y, x), which causes errors if the selectors in question are used. This should be changed to use math_atan2 as specified in the Source docs.

[martin-henz]

I found various other references to math_atan, instead of math_atan2:

	modified:   xml/chapter2/section4/subsection2.xml
	modified:   xml/chapter2/section4/subsection3.xml
	modified:   xml/chapter2/section5/subsection2.xml

[l-armstrong]

Is this issue still open to being work on?