* Operation `--delete-generations' to delete generations of a

profile.  Arguments are either generation number, or `old' to delete
  all non-current generations.  Typical use:

  $ nix-env --delete-generations old
  $ nix-collect-garbage

* istringstream -> string2Int.

src/libutil/util.cc

@@ -616,3 +616,11 @@ bool statusOk(int status)
 {
     return WIFEXITED(status) && WEXITSTATUS(status) == 0;
 }
+
+
+bool string2Int(const string & s, int & n)
+{
+    istringstream str(s);
+    str >> n;
+    return str && str.eof();
+}