Index: node_modules/d3-polygon/src/cross.js
===================================================================
--- node_modules/d3-polygon/src/cross.js	(revision e4c61dd6cd86e06265bc2bd91adba84a0f04044a)
+++ node_modules/d3-polygon/src/cross.js	(revision e4c61dd6cd86e06265bc2bd91adba84a0f04044a)
@@ -0,0 +1,7 @@
+// Returns the 2D cross product of AB and AC vectors, i.e., the z-component of
+// the 3D cross product in a quadrant I Cartesian coordinate system (+x is
+// right, +y is up). Returns a positive value if ABC is counter-clockwise,
+// negative if clockwise, and zero if the points are collinear.
+export default function(a, b, c) {
+  return (b[0] - a[0]) * (c[1] - a[1]) - (b[1] - a[1]) * (c[0] - a[0]);
+}