From 709ece335f1c134bca4de102a562549282087730 Mon Sep 17 00:00:00 2001
From: Anders Olsson <anders.e.olsson@gmail.com>
Date: Fri, 24 Apr 2026 07:00:26 +0200
Subject: [PATCH] feat: introduce InferenceError; mu_sigma panic already
 eliminated

mu_sigma was deleted as part of the Gaussian nat-param rewrite (its
only callers were the old Mul/Div impls). This commit adds the
InferenceError enum as a seed for the T2 API surface, with the
NegativePrecision variant that mu_sigma would have returned.

Part of T0 engine redesign.
---
 src/error.rs | 18 ++++++++++++++++++
 src/lib.rs   |  2 ++
 2 files changed, 20 insertions(+)
 create mode 100644 src/error.rs

diff --git a/src/error.rs b/src/error.rs
new file mode 100644
index 0000000..3886451
--- /dev/null
+++ b/src/error.rs
@@ -0,0 +1,18 @@
+use std::fmt;
+
+#[derive(Debug, Clone, PartialEq)]
+pub enum InferenceError {
+    NegativePrecision { pi: f64 },
+}
+
+impl fmt::Display for InferenceError {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Self::NegativePrecision { pi } => {
+                write!(f, "precision must be non-negative; got {pi}")
+            }
+        }
+    }
+}
+
+impl std::error::Error for InferenceError {}
diff --git a/src/lib.rs b/src/lib.rs
index c761f08..9579032 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -11,6 +11,7 @@ pub mod agent;
 mod approx;
 pub mod batch;
 pub mod drift;
+mod error;
 mod game;
 pub mod gaussian;
 mod history;
@@ -19,6 +20,7 @@ mod message;
 pub mod player;
 
 pub use drift::{ConstantDrift, Drift};
+pub use error::InferenceError;
 pub use game::Game;
 pub use gaussian::Gaussian;
 pub use history::History;