Why not?

src/cmd/map/libmap/twocirc.c

@@ -0,0 +1,80 @@
+#include <u.h>
+#include <libc.h>
+#include "map.h"
+
+static double
+quadratic(double a, double b, double c)
+{
+	double disc = b*b - 4*a*c;
+	return disc<0? 0: (-b-sqrt(disc))/(2*a);
+}
+
+/* for projections with meridians being circles centered
+on the x axis and parallels being circles centered on the y
+axis.  Find the intersection of the meridian thru (m,0), (90,0),
+with the parallel thru (0,p), (p1,p2) */
+
+static int
+twocircles(double m, double p, double p1, double p2, double *x, double *y)
+{
+	double a;	/* center of meridian circle, a>0 */
+	double b;	/* center of parallel circle, b>0 */
+	double t,bb;
+	if(m > 0) {
+		twocircles(-m,p,p1,p2,x,y);
+		*x = -*x;
+	} else if(p < 0) {
+		twocircles(m,-p,p1,-p2,x,y);
+		*y = -*y;
+	} else if(p < .01) {
+		*x = m;
+		t = m/p1;
+		*y = p + (p2-p)*t*t;
+	} else if(m > -.01) {
+		*y = p;
+		*x = m - m*p*p;
+	} else {
+		b = p>=1? 1: p>.99? 0.5*(p+1 + p1*p1/(1-p)):
+			0.5*(p*p-p1*p1-p2*p2)/(p-p2);
+		a = .5*(m - 1/m);
+		t = m*m-p*p+2*(b*p-a*m);
+		bb = b*b;
+		*x = quadratic(1+a*a/bb, -2*a + a*t/bb,
+			t*t/(4*bb) - m*m + 2*a*m);
+		*y = (*x*a+t/2)/b;
+	}
+	return 1;
+}		
+
+static int
+Xglobular(struct place *place, double *x, double *y)
+{
+	twocircles(-2*place->wlon.l/PI,
+		2*place->nlat.l/PI, place->nlat.c, place->nlat.s, x, y);
+	return 1;
+}	
+
+proj
+globular(void)
+{
+	return Xglobular;
+}
+
+static int
+Xvandergrinten(struct place *place, double *x, double *y)
+{
+	double t = 2*place->nlat.l/PI;
+	double abst = fabs(t);
+	double pval = abst>=1? 1: abst/(1+sqrt(1-t*t));
+	double p2 = 2*pval/(1+pval);
+	twocircles(-place->wlon.l/PI, pval, sqrt(1-p2*p2), p2, x, y);
+	if(t < 0) 
+		*y = -*y;
+	return 1;
+}
+
+proj
+vandergrinten(void)
+{
+	return Xvandergrinten;
+}