convenience functions zforward_dir and zpush0

3d-models.cpp

@@ -615,6 +615,7 @@ void geometry_information::animate_bird(hpcshape& orig, hpcshape_animated& anima
   }
 
 EX hyperpoint forward_dir(ld x) { return prod ? point3(x, 0, 0) : xpush0(x); }
+EX hyperpoint zforward_dir(ld z) { return prod ? point3(0, 0, z) : zpush0(z); }
 
 EX hyperpoint dir_to_point(hyperpoint h) { return prod ? product::direct_exp(h) : h; }