DOC: explain usage of argument() function

docs/usage/sympy.ipynb

@@ -213,6 +213,72 @@
     "Math(aslatex({e: e.doit() for e in exprs}))"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "By default, instance attributes are converted ['sympified'](https://docs.sympy.org/latest/modules/core.html#module-sympy.core.sympify). To avoid this behavior, use the {func}`.argument` function."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "from typing import Callable\n",
+    "\n",
+    "from ampform.sympy import argument\n",
+    "\n",
+    "\n",
+    "class Transformation:\n",
+    "    def __init__(self, power: int) -> None:\n",
+    "        self.power = power\n",
+    "\n",
+    "    def __call__(self, x: sp.Basic, y: sp.Basic) -> sp.Expr:\n",
+    "        return x + y**self.power\n",
+    "\n",
+    "\n",
+    "@unevaluated\n",
+    "class MyExpr(sp.Expr):\n",
+    "    x: Any\n",
+    "    y: Any\n",
+    "    functor: Callable = argument(sympify=False)\n",
+    "\n",
+    "    def evaluate(self) -> sp.Expr:\n",
+    "        return self.functor(self.x, self.y)"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Notice how the `functor` attribute has not been sympified (there is no SymPy equivalent for a callable object), but the `functor` can be called in the `evaluate()`/`doit()` method."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "a, b, k = sp.symbols(\"a b k\")\n",
+    "expr = MyExpr(a, y=b, functor=Transformation(power=k))\n",
+    "assert expr.x is a\n",
+    "assert expr.y is b\n",
+    "assert not isinstance(expr.functor, sp.Basic)\n",
+    "Math(aslatex({expr: expr.doit()}))"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    ":::{tip}\n",
+    "An example where this is used, is in the {class}`.EnergyDependentWidth` class, where we do not want to sympify the {attr}`~.EnergyDependentWidth.phsp_factor` protocol.\n",
+    ":::"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},