Correct two type-checking errors.

Author: eloj

README.md

@@ -1,8 +1,6 @@
 # Approximate Shortest Superstring Generator
 
-_WARNING: While this note persist, I may force-push to master_
-
-A Python library and tool for generating approximate [shortest common superstrings](https://en.wikipedia.org/wiki/Shortest_common_supersequence#Shortest_common_superstring).
+A Python tool for generating approximate [shortest common superstrings](https://en.wikipedia.org/wiki/Shortest_common_supersequence#Shortest_common_superstring).
 
 All code is provided under the [MIT License](LICENSE).
 
@@ -142,7 +140,7 @@ case it's 'brutedp'. If you specify an invalid name here, you should get a list
 `make_substring_free(list) -> list`
 : For Greedy to work as originally specified, its input must be _substring-free_ (aka _factor-free_), i.e contain no elements that are substrings of one another. This function will process a list to ensure this pre-condition is true.
 
-See the [ssp.py source code](ssp.py) for details.
+See the [ssp.py source code](ssp.py) for details and references.
 
 ### Example API Usage
 
@@ -157,7 +155,6 @@ res = ssp.generate_superstring(arr)
 
 ## TODO
 
-* Verify that the type-spec in ssp.py is actually correct.
 * Implement encoding into bitstrings.
 * Built-in benchmark.
 

ssp.py

@@ -169,13 +169,13 @@ def overlap(a, b):
                 return len(a) - start
             start += 1
 
-    shortest = None
+    shortest = ""
     for perm in permutations(arr):
         sup = perm[0]
         for i in range(len(arr) - 1):
             olen = overlap(perm[i], perm[i+1])
             sup += perm[i+1][olen:]
-        if shortest is None or len(sup) < len(shortest):
+        if shortest == "" or len(sup) < len(shortest):
             shortest = sup
     return shortest
 
@@ -258,6 +258,7 @@ def brutedijkstra(arr: list[str]) -> str:
             if not updated:
                 return candidate
         del paths[min_len]
+    return ""
 
 def generate_superstring(strings: list[str], func=greedy) -> str:
     """