Why not?

src/cmd/map/libmap/bicentric.c

@@ -0,0 +1,25 @@
+#include <u.h>
+#include <libc.h>
+#include "map.h"
+
+static struct coord center;
+
+static int
+Xbicentric(struct place *place, double *x, double *y)
+{
+	if(place->wlon.c<=.01||place->nlat.c<=.01)
+		return(-1);
+	*x = -center.c*place->wlon.s/place->wlon.c;
+	*y = place->nlat.s/(place->nlat.c*place->wlon.c);
+	return(*x**x+*y**y<=9);
+}
+
+proj
+bicentric(double l)
+{
+	l = fabs(l);
+	if(l>89)
+		return(0);
+	deg2rad(l,&center);
+	return(Xbicentric);
+}