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diff --git a/octave_packages/nurbs-1.3.6/private/nrb_srf_basisfun_der__.m b/octave_packages/nurbs-1.3.6/private/nrb_srf_basisfun_der__.m
new file mode 100644
index 0000000..9524f91
--- /dev/null
+++ b/octave_packages/nurbs-1.3.6/private/nrb_srf_basisfun_der__.m
@@ -0,0 +1,52 @@
+function [Bu, Bv, N] = nrb_srf_basisfun_der__ (points, nrb);
+
+%  __NRB_SRF_BASISFUN_DER__: Undocumented internal function
+%
+%   Copyright (C) 2009 Carlo de Falco
+%   This software comes with ABSOLUTELY NO WARRANTY; see the file
+%   COPYING for details.  This is free software, and you are welcome
+%   to distribute it under the conditions laid out in COPYING.
+
+  m    = size (nrb.coefs, 2) -1;
+  n    = size (nrb.coefs, 3) -1;
+  
+  p    = nrb.order(1) -1;
+  q    = nrb.order(2) -1;
+  
+  u = points(1,:);
+  v = points(2,:);
+  npt = length(u);
+  
+  U    = nrb.knots{1};
+  V    = nrb.knots{2};
+  
+  w    = squeeze(nrb.coefs(4,:,:));
+  
+  spu  =  findspan (m, p, u, U); 
+  spv  =  findspan (n, q, v, V);
+  N    =  nrbnumbasisfun (points, nrb);
+
+  NuIkuk = basisfun (spu, u, p, U); 
+  NvJkvk = basisfun (spv, v, q, V);
+  
+  NuIkukprime = basisfunder (spu, p, u, U, 1);
+  NuIkukprime = reshape (NuIkukprime(:,2,:), npt, []);
+  
+  NvJkvkprime = basisfunder (spv, q, v, V, 1);
+  NvJkvkprime = reshape (NvJkvkprime(:,2,:), npt, []);
+  
+  for k=1:npt
+    wIkaJkb(1:p+1, 1:q+1) = reshape (w(N(k, :)), p+1, q+1);
+    
+    Num    = (NuIkuk(k, :).' * NvJkvk(k, :)) .* wIkaJkb;
+    Num_du = (NuIkukprime(k, :).' * NvJkvk(k, :)) .* wIkaJkb;
+    Num_dv = (NuIkuk(k, :).' * NvJkvkprime(k, :)) .* wIkaJkb;
+    Denom  = sum(sum(Num));
+    Denom_du = sum(sum(Num_du));
+    Denom_dv = sum(sum(Num_dv));
+    
+    Bu(k, :) = reshape((Num_du/Denom - Denom_du.*Num/Denom.^2),1,[]);
+    Bv(k, :) = reshape((Num_dv/Denom - Denom_dv.*Num/Denom.^2),1,[]);
+  end
+  
+end
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