--- a/scripts/set/powerset.m
+++ b/scripts/set/powerset.m
@@ -19,11 +19,17 @@
 ## -*- texinfo -*-
 ## @deftypefn  {Function File} {} powerset (@var{a})
 ## @deftypefnx {Function File} {} powerset (@var{a}, "rows")
+## Compute the powerset (all subsets) of the set @var{a}.
 ##
-## Return a cell array containing all subsets of the set @var{a}.
+## The set @var{a} must be a numerical matrix or a cell array of strings.  The
+## output will always be a cell array of either vectors or strings.
 ##
+## With the optional second argument @code{"rows"}, each row of the set @var{a}
+## is considered one element of the set.  As a result, @var{a} must then be a
+## numerical 2D matrix.
+##
+## @seealso{unique, union, setxor, setdiff, ismember}
 ## @end deftypefn
-## @seealso{unique, union, setxor, setdiff, ismember}
 
 function p = powerset (a, byrows_arg)
 
@@ -31,7 +37,7 @@
 
   if (nargin == 2)
     if (! strcmpi (byrows_arg, "rows"))
-      error ('powerset: expecting third argument to be "rows"');
+      error ('powerset: expecting second argument to be "rows"');
     elseif (iscell (a))
       warning ('powerset: "rows" not valid for cell arrays');
     else
@@ -40,6 +46,9 @@
   elseif (nargin != 1)
     print_usage ();
   endif
+  if (iscell (a) && ! iscellstr (a))
+    error ("powerset: cell arrays can only used for character strings");
+  endif
 
   if (byrows)
     a = unique (a, byrows_arg);
@@ -50,7 +59,7 @@
   endif
 
   if (n == 0)
-    p = [];
+    p = {};
   else
     if (n > 32)
       error ("powerset: not implemented for more than 32 elements");
@@ -77,8 +86,12 @@
 endfunction
 
 
-%!test
+%!shared c, p
 %! c = sort (cellstr ({ [], [1], [2], [3], [1, 2], [1, 3], [2, 3], [1, 2, 3]}));
 %! p = sort (cellstr (powerset ([1, 2, 3])));
-%! assert (p, c);
+%!assert (p, c);
+%! c = sort (cellstr ({ [], [1:3], [2:4], [3:5], [1:3; 2:4], [1:3; 3:5], [2:4; 3:5], [1:3; 2:4; 3:5]}));
+%! p = sort (cellstr (powerset ([1:3;2:4;3:5], "rows")));
+%!assert (p,c);
+%!assert (powerset([]), {});  # always return a cell array