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diff --git a/src/math/atan2.go b/src/math/atan2.go
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--- /dev/null
+++ b/src/math/atan2.go
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+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package math
+
+// Atan2 returns the arc tangent of y/x, using
+// the signs of the two to determine the quadrant
+// of the return value.
+//
+// Special cases are (in order):
+//	Atan2(y, NaN) = NaN
+//	Atan2(NaN, x) = NaN
+//	Atan2(+0, x>=0) = +0
+//	Atan2(-0, x>=0) = -0
+//	Atan2(+0, x<=-0) = +Pi
+//	Atan2(-0, x<=-0) = -Pi
+//	Atan2(y>0, 0) = +Pi/2
+//	Atan2(y<0, 0) = -Pi/2
+//	Atan2(+Inf, +Inf) = +Pi/4
+//	Atan2(-Inf, +Inf) = -Pi/4
+//	Atan2(+Inf, -Inf) = 3Pi/4
+//	Atan2(-Inf, -Inf) = -3Pi/4
+//	Atan2(y, +Inf) = 0
+//	Atan2(y>0, -Inf) = +Pi
+//	Atan2(y<0, -Inf) = -Pi
+//	Atan2(+Inf, x) = +Pi/2
+//	Atan2(-Inf, x) = -Pi/2
+func Atan2(y, x float64) float64
+
+func atan2(y, x float64) float64 {
+	// special cases
+	switch {
+	case IsNaN(y) || IsNaN(x):
+		return NaN()
+	case y == 0:
+		if x >= 0 && !Signbit(x) {
+			return Copysign(0, y)
+		}
+		return Copysign(Pi, y)
+	case x == 0:
+		return Copysign(Pi/2, y)
+	case IsInf(x, 0):
+		if IsInf(x, 1) {
+			switch {
+			case IsInf(y, 0):
+				return Copysign(Pi/4, y)
+			default:
+				return Copysign(0, y)
+			}
+		}
+		switch {
+		case IsInf(y, 0):
+			return Copysign(3*Pi/4, y)
+		default:
+			return Copysign(Pi, y)
+		}
+	case IsInf(y, 0):
+		return Copysign(Pi/2, y)
+	}
+
+	// Call atan and determine the quadrant.
+	q := Atan(y / x)
+	if x < 0 {
+		if q <= 0 {
+			return q + Pi
+		}
+		return q - Pi
+	}
+	return q
+}