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diff --git a/plug-ins/selection-to-path/vector.h b/plug-ins/selection-to-path/vector.h
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+/* vector.h: operations on vectors and points.
+ *
+ * Copyright (C) 1992 Free Software Foundation, Inc.
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3, or (at your option)
+ * any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#ifndef VECTOR_H
+#define VECTOR_H
+
+#include "types.h"
+
+/* Our vectors are represented as displacements along the x and y axes.  */
+
+typedef struct
+{
+  real dx, dy;
+} vector_type;
+
+
+/* Consider a point as a vector from the origin.  */
+extern vector_type make_vector (const real_coordinate_type);
+
+/* And a vector as a point, i.e., a displacement from the origin.  */
+extern real_coordinate_type vector_to_point (const vector_type);
+
+
+/* Definitions for these common operations can be found in any decent
+   linear algebra book, and most calculus books.  */
+
+extern real magnitude (const vector_type);
+extern vector_type normalize (const vector_type);
+
+extern vector_type Vadd (const vector_type, const vector_type);
+extern real Vdot (const vector_type, const vector_type);
+extern vector_type Vmult_scalar (const vector_type, const real);
+extern real Vangle (const vector_type in, const vector_type out);
+
+/* These operations could have been named `P..._vector' just as well as
+   V..._point, so we may as well allow both names.  */
+#define Padd_vector Vadd_point
+extern real_coordinate_type Vadd_point
+  (const real_coordinate_type, const vector_type);
+
+#define Psubtract_vector Vsubtract_point
+extern real_coordinate_type Vsubtract_point
+  (const real_coordinate_type, const vector_type);
+
+/* This returns the rounded sum.  */
+#define IPadd_vector Vadd_int_point
+extern coordinate_type Vadd_int_point
+  (const coordinate_type, const vector_type);
+
+/* Take the absolute value of both components.  */
+extern vector_type Vabs (const vector_type);
+
+
+/* Operations on points with real coordinates.  It is not orthogonal,
+   but more convenient, to have the subtraction operator return a
+   vector, and the addition operator return a point.  */
+extern vector_type Psubtract
+  (const real_coordinate_type, const real_coordinate_type);
+
+/* These are heavily used in spline fitting, so we define them as macros
+   instead of functions.  */
+#define Padd(rc1, rc2)							\
+  ((real_coordinate_type) { (rc1).x + (rc2).x, (rc1).y + (rc2).y })
+#define Pmult_scalar(rc, r)						\
+  ((real_coordinate_type) { (rc).x * (r), (rc).y * (r) })
+
+/* Similarly, for points with integer coordinates; here, a subtraction
+   operator that does return another point is useful.  */
+extern vector_type IPsubtract
+  (const coordinate_type, const coordinate_type);
+extern coordinate_type IPsubtractP
+  (const coordinate_type, const coordinate_type);
+extern coordinate_type IPadd
+  (const coordinate_type, const coordinate_type);
+extern coordinate_type IPmult_scalar (const coordinate_type, const int);
+extern real_coordinate_type IPmult_real
+  (const coordinate_type, const real);
+extern boolean IPequal (const coordinate_type, const coordinate_type);
+
+#endif /* not VECTOR_H */