+Value builtin_fn(Arguments arguments, const Location& loc)
+{
+  Parameters parameters = Parameters::parse(std::move(arguments), loc, {"r"}, {"d"});
+  parameters.set_caller("fn");
+
+  auto name = parameters.exactlyOneOf({ "r", "d" });
+  if (!name) {
+    return Value::undefined.clone();
+  }
+
+  double v;
+  if (!parameters.validate_number(*name, v)) {
+    return Value::undefined.clone();
+  }
+
+  if (*name == "d") {
+    v = v / 2;
+  }
+
+  auto fn = parameters["$fn"].toDouble();
+  auto fs = parameters["$fs"].toDouble();
+  auto fa = parameters["$fa"].toDouble();
+
+  return Calc::get_fragments_from_r(v, fn, fs, fa);
+}