diff --git a/case_study/assertion.py b/case_study/assertion.py
index 2767b65..fe6f6b9 100644
--- a/case_study/assertion.py
+++ b/case_study/assertion.py
@@ -1,3 +1,5 @@
+from typing import Any
+
 from sympy import Symbol, sympify
 
 
@@ -17,7 +19,7 @@ class Assertion:
             Any unknown symbol within the expression is defined as sympy.Symbol and is expected to match a variable.
     """
 
-    ns = {}
+    ns: dict[str, Any] = {}
 
     def __init__(self, expr: str):
         self._expr = Assertion.do_sympify(expr)
diff --git a/pyproject.toml b/pyproject.toml
index f65fe08..b755ff7 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -24,6 +24,7 @@ build-backend = "setuptools.build_meta"
         "fmpy>=0.3.21",
         "matplotlib>=3.7.1",
         "pint>=0.24.3",
+        "simpy>=4.1.1"
     ]
     #dynamic = ["version"]