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diff --git a/proofsearch/include/atp/core/literal.h b/proofsearch/include/atp/core/literal.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 literal.h
+/// @brief A literal is a (possibly negated) atomic formula.
+
+#include "atp/core/types.h"
+
+namespace atp {
+
+/// A literal: a polarity flag plus the TermId of the atom.
+struct Literal {
+    TermId atom_;       ///< TermId of the predicate application
+    bool is_positive_;  ///< true = positive, false = negated
+
+    /// Complement: flip polarity, same atom.
+    [[nodiscard]] Literal complement() const { return {.atom_ = atom_, .is_positive_ = !is_positive_}; }
+
+    bool operator==(const Literal& other) const = default;
+};
+
+}  // namespace atp