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+* Routines for selecting the k largest or smallest values could use
+heapsort for speed O(N log k) rather than insertion O(N k).
+
+* Sorting of complex arrarys without using additional memory. We try
+to avoid allocating memory internally in GSL, so internally computing
+the magnitudes and storing them in a temporary array is undesirable. 
+
+Obviously a complex array can be sorted using sqrt(x*x + y*y) <=>
+sqrt(u*u + v*v) (written in a numerically stable way) for every
+comparison, but this may be unacceptably slow. Maybe not? It is just a
+constant factor. The square roots could sometimes be avoided by
+optimization,
+
+    (x,y) = (MAX(|x|,|y|), MIN(|x|,|y|))
+    (u,v) = (MAX(|u|,|v|), MIN(|u|,|v|))
+
+    if (x < u/sqrt(2))     /* This part is optional optimization */
+       return -1 
+    if (x > u*sqrt(2))
+       return +1
+
+    if (x == 0 && u == 0) ...
+    if (x == 0) ...
+    if (u == 0) ...    
+
+    t = u*sqrt((1+(v/u)^2)/(1+(y/x)^2))
+    
+    if (x < t)
+       return -1
+    if (x > t)
+       return +1 
+    else
+       return 0
+
+but this does depend on the data having sufficient range for the
+optimization to be worthwhile, otherwise it is an extra cost.