Arctan2​(PeterO.​Numbers.​EDecimal,​PeterO.​Numbers.​EDecimal,​PeterO.​Numbers.​EContext)

A zero is a zero, whatever its sign.Math.Atan2 reads a negative zero
as a limit from below and answers -pi on the negative axis for it, and this
did the same. A decimal's negative zero is not a limit: it is what
0E-100 * -0.43 multiplies to, an artefact of the exponent arithmetic, and
reading -pi off it put -1.54 - 0i a hair below the negative axis, so
that (-1.54)^(-1/2) came back +0.80 i with the downcasting off and
-0.80 i with it on -- the setting deciding a branch. A number on the negative
axis has argument pi. EDecimal.Sign is 0 for either zero, and is what
is read here. https://github.com/asc-community/AngouriMath/issues/1378