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Four-quadrant inverse tangent

`P = atan2(`

returns the
four-quadrant inverse
tangent (tan`Y`

,`X`

)^{-1}) of `Y`

and
`X`

, which must be real. The `atan2`

function follows the convention that `atan2(x,x)`

returns
`0`

when `x`

is mathematically zero (either
`0`

or `-0`

).