`cot alpha = cos alpha / sin alpha`
`cot alpha = 1/(tan alpha)`
`cot alpha = - tan(alpha + pi/2) = - tan(alpha - pi/2)`
`cot alpha = sqrt(1/sin^2alpha - 1)`
`-cot alpha = cot(-alpha)`
`(cot alpha)^' =-1/sin^2 alpha`
`int cot(alpha)dalpha = -ln |sin alpha| + C`