Why not?

src/cmd/map/libmap/elliptic.c

@@ -0,0 +1,35 @@
+#include <u.h>
+#include <libc.h>
+#include "map.h"
+
+struct coord center;
+
+static int
+Xelliptic(struct place *place, double *x, double *y)
+{
+	double r1,r2;
+	r1 = acos(place->nlat.c*(place->wlon.c*center.c
+		- place->wlon.s*center.s));
+	r2 = acos(place->nlat.c*(place->wlon.c*center.c
+		+ place->wlon.s*center.s));
+	*x = -(r1*r1 - r2*r2)/(4*center.l);
+	*y = (r1*r1+r2*r2)/2 - (center.l*center.l+*x**x);
+	if(*y < 0)
+		*y = 0;
+	*y = sqrt(*y);
+	if(place->nlat.l<0)
+		*y = -*y;
+	return(1);
+}
+
+proj
+elliptic(double l)
+{
+	l = fabs(l);
+	if(l>89)
+		return(0);
+	if(l<1)
+		return(Xazequidistant);
+	deg2rad(l,&center);
+	return(Xelliptic);
+}