pop(): An arbitrary element is removed, not a random element.

Doc/lib/libsets.tex

@@ -143,7 +143,8 @@ but not found in \class{ImmutableSet}:
          {Removes element \var{x} from set \var{s} like \var{s}.remove(\var{x})
           but does not raise a KeyError if \var{x} is not in \var{s}}
   \lineii{\var{s}.pop()}
-         {Remove and return a randomly-chosen element from \var{s}}
+         {Remove and return an element from \var{s}; no guarantee is
+          made about which element is removed}
   \lineii{\var{s}.update(\var{t})}
          {Add elements from \var{t} to set \var{s}}
   \lineii{\var{s}.clear()}