Update the class definitions

algorithms.py

@@ -1,6 +1,8 @@
 """
 Python version of the simulation algorithm.
 """
+from __future__ import annotations
+
 import argparse
 import dataclasses
 import heapq
@@ -111,23 +113,25 @@ def find(self, v):
         return j + 1
 
 
+# Once we drop support for 3.9 we can use slots=True to prevent
+# writing extra attrs.
+@dataclasses.dataclass  # (slots=True)
 class Segment:
     """
     A class representing a single segment. Each segment has a left
     and right, denoting the loci over which it spans, a node and a
     next, giving the next in the chain.
     """
 
-    def __init__(self, index):
-        self.left = None
-        self.right = None
-        self.node = None
-        self.prev = None
-        self.next = None
-        self.index = index
-        self.population = None
-        self.label = 0
-        self.lineage = None
+    index: int
+    left: float = 0
+    right: float = 0
+    node: int = -1
+    prev: Segment = None
+    next: Segment = None  # noqa: A003
+    lineage: Lineage = None
+    population: int = -1
+    label: int = 0
 
     def __repr__(self):
         return repr((self.left, self.right, self.node))
@@ -1070,7 +1074,6 @@ def alloc_segment(
         prev=None,
         next=None,  # noqa: A002
         label=0,
-        hull=None,
     ):
         """
         Pops a new segment off the stack and sets its properties.
@@ -1083,7 +1086,6 @@ def alloc_segment(
         s.next = next
         s.prev = prev
         s.label = label
-        s.hull = hull
         return s
 
     def alloc_lineage(self, head):