diff --git a/command_prompt.py b/command_prompt.py
index f935a8c..23342dc 100644
--- a/command_prompt.py
+++ b/command_prompt.py
@@ -4,8 +4,6 @@ from kivy.graphics import Color,Rectangle
 import glob
 import os.path
 
-from tools import common_substr
-
 class Command_prompt(Label):
     
     def __init__(self,app,**kwargs):
@@ -272,4 +270,4 @@ class Command_prompt(Label):
                     name="\""+name+"\""
                 self.app.status_bar.raw_text+=name+" "
             self.app.status_bar.draw()
-            return common_substr(paths)[end:]
+            return os.path.commonprefix(paths)[end:]
diff --git a/tools.py b/tools.py
index 06e36a0..a9ef4e9 100644
--- a/tools.py
+++ b/tools.py
@@ -9,16 +9,3 @@ def isint_nonzero(x):
     if abs(x)<1e-11:
         return False
     return abs(round(x)-x)<1e-11
-
-# find the common substring in a list of strings
-def common_substr(strings):
-    out=""
-    if len(strings)==0:
-        return out
-    for i in range(len(strings[0])):
-        for j in range(1,len(strings)):
-            if strings[j][i]!=strings[0][i]:
-                return out
-        out+=strings[0][i]
-
-    return out