diff --git a/tools/lib/graph.py b/tools/lib/graph.py
new file mode 100644
index 0000000000..e25f2c2296
--- /dev/null
+++ b/tools/lib/graph.py
@@ -0,0 +1,67 @@
+from collections import defaultdict
+
+from typing import List, Set, Tuple
+
+class Graph(object):
+    def __init__(self, *tuples):
+        # type: (Tuple[str, str]) -> None
+        self.children = defaultdict(list) # type: defaultdict[str, List[str]]
+        self.parents = defaultdict(list) # type: defaultdict[str, List[str]]
+        self.nodes = set() # type: Set[str]
+
+        for parent, child in tuples:
+            self.parents[child].append(parent)
+            self.children[parent].append(child)
+            self.nodes.add(parent)
+            self.nodes.add(child)
+
+    def remove_exterior_nodes(self):
+        # type: () -> None
+        still_work_to_do = True
+        while still_work_to_do:
+            still_work_to_do = False # for now
+            for node in self.nodes:
+                if self.parents[node] and self.children[node]:
+                    # we are an interior node still
+                    continue
+                self.remove(node)
+                still_work_to_do = True
+                break
+
+    def remove(self, node):
+        # type: (str) -> None
+        for parent in self.parents[node]:
+            self.children[parent].remove(node)
+        for child in self.children[node]:
+            self.parents[child].remove(node)
+        self.nodes.remove(node)
+
+def make_dot_file(graph):
+    # type: (Graph) -> str
+    buffer = 'digraph G {\n'
+    for node in graph.nodes:
+        buffer += node + ';\n'
+        for child in graph.children[node]:
+            buffer += '{} -> {};\n'.format(node, child)
+    buffer += '}'
+    return buffer
+
+def test():
+    # type: () -> None
+    graph = Graph(
+        ('x', 'a'),
+        ('a', 'b'),
+        ('b', 'c'),
+        ('c', 'a'),
+        ('c', 'd'),
+        ('d', 'e'),
+        ('e', 'f'),
+        ('e', 'g'),
+    )
+    graph.remove_exterior_nodes()
+
+    s = make_dot_file(graph)
+    open('zulip-deps.dot', 'w').write(s)
+
+if __name__ == '__main__':
+    test()