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+// Copyright (C) 2013, Gabriel Dos Reis.
+// All rights reserved.
+// Written by Gabriel Dos Reis.
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     - Redistributions of source code must retain the above copyright
+//       notice, this list of conditions and the following disclaimer.
+//
+//     - Redistributions in binary form must reproduce the above copyright
+//       notice, this list of conditions and the following disclaimer in
+//       the documentation and/or other materials provided with the
+//       distribution.
+//
+//     - Neither the name of OpenAxiom, nor the names of its contributors may
+//       be used to endorse or promote products derived from this software
+//       without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS
+// IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+// TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A
+// PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER
+// OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+// EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+// PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+// LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+// NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+// SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#ifndef OPENAXIOM_STRUCTURE_included
+#define OPENAXIOM_STRUCTURE_included
+
+#include <iterator>
+
+namespace OpenAxiom {
+   // -- helper classes for structural abstractions --
+   namespace structure {
+      // unary structures
+      template<typename T>
+      struct unary {
+         explicit constexpr unary(const T& t) : arg(t) { };
+         constexpr const T& operand() const { return arg; }
+      private:
+         const T arg;
+      };
+
+      // binary structures
+      template<typename T, typename U = T>
+      struct binary {
+         constexpr binary(const T& t, const U& u) : arg0(t), arg1(u) { };
+         constexpr const T& first() const { return arg0; }
+         constexpr const U& second() const { return arg1; }
+      private:
+         const T arg0;
+         const U arg1;
+      };
+
+      // ternary structures
+      template<typename T, typename U = T, typename V = U>
+      struct ternary {
+         constexpr ternary(const T& t, const U& u, const V& v)
+               : arg0(t), arg1(u), arg2(v) { }
+         constexpr const T& first() const { return arg0; }
+         constexpr const U& second() const { return arg1; }
+         constexpr const V& third() const { return arg2; }
+      private:
+         const T arg0;
+         const U arg1;
+         const V arg2;
+      };
+   }
+}
+
+#endif  // OPENAXIOM_STRUCTURE_included