Add explainer for not applicable inputs.

Author: MImmesberger

docs/tutorials/simple_example.ipynb

@@ -288,7 +288,20 @@
     "# Edit the leafs in the template and then read it back in\n",
     "with PATH_FOR_TEMPLATE.open(\"r\") as f:\n",
     "    MAPPER = yaml.safe_load(f)\n",
-    "```"
+    "```\n",
+    "\n",
+    "Some inputs in the mapper may not be directly relevant to an individual’s current\n",
+    "situation, as they depend on future circumstances or choices. For example, variables\n",
+    "such as `jahr_renteneintritt` or `alter_beginn_leistungsbezug_sonstige_private_vorsorge`\n",
+    "are only meaningful for individuals who actually receive pensions. However, since\n",
+    "GETTSIM represents the tax and transfer system as a static directed acyclic graph (DAG),\n",
+    "these inputs are still required, even when they are not applicable.  \n",
+    "\n",
+    "In such cases, you can safely assign a reasonable default value. The exact number\n",
+    "usually does not matter (as long as the corresponding benefit, such as the pension, is\n",
+    "set to zero), but it must still be a valid input. For instance, `jahr_renteneintritt`\n",
+    "should not be set to 0 or `None`, but any plausible year value will work.  \n",
+    "\n"
    ]
   },
   {