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diff --git a/include/atp/core/clause.h b/include/atp/core/clause.h
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+/*
+ * richard-atp-engine: A saturation-based automated theorem prover for first-order logic.
+ * Copyright (C) 2026 Richard Qin
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+
+#pragma once
+
+/// @file clause.h
+/// @brief A clause is a disjunction of literals, with provenance tracking.
+///
+/// NOTE: V1 uses std::vector<Literal> for simplicity.
+/// Future optimization: migrate to a Clause Arena with contiguous storage.
+
+#include <vector>
+
+#include "atp/core/literal.h"
+#include "atp/core/types.h"
+
+namespace atp {
+
+/// A clause: a disjunction of literals plus provenance for proof reconstruction.
+struct Clause {
+    ClauseId id = kInvalidId;
+    std::vector<Literal> literals;
+
+    // Provenance (for proof trace)
+    InferenceRule rule = InferenceRule::kInput;
+    ClauseId parent1 = kInvalidId;
+    ClauseId parent2 = kInvalidId;
+    uint16_t depth = 0;  ///< Derivation depth (for BFS scheduling)
+
+    /// True if the clause has no literals (contradiction).
+    [[nodiscard]] bool isEmpty() const { return literals.empty(); }
+
+    /// Number of literals.
+    [[nodiscard]] size_t size() const { return literals.size(); }
+};
+
+}  // namespace atp