Arctan​(PeterO.​Numbers.​EDecimal,​PeterO.​Numbers.​EContext)

Reduced to [0, 1] by arctan(x) = pi/2 - arctan(1/x), then to within
1/128 of zero by the nearest c = j/64 -- arctan x = arctan c +
arctan((x - c)/(1 + xc)), with arctan c from the constant cache -- where
the series y - y^3/3 + y^5/5 - ... is thirty terms at a hundred digits, each
a multiplication and a division by a small integer. It was halved to a twentieth
by arctan(x) = 2 arctan(x/(1 + sqrt(1 + x^2))), three square roots and
forty-seven terms; before that the arcsine of x/sqrt(1 + x^2), two
milliseconds at a hundred digits for an argument of a third.
https://github.com/asc-community/AngouriMath/issues/1338