+ """
+ Find a free EGRE key
+ """
+ def free_egre_key(self):
+ used = set()
+ for tag in self.getSliceTags():
+ if tag.tagname == 'egre_key':
+ used.add(int(tag.value))
+
+ for i in range(1, 256):
+ if i not in used:
+ key = i
+ break
+ else:
+ raise KeyError("No more EGRE keys available")
+
+ return str(key)
+