text: ///
  /// BSD 3-Clause License
  ///
  /// Copyright (c) 2020, Alexandre Arsenault
  /// All rights reserved.
  ///
  /// 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 the copyright holder 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 HOLDER 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.
  ///
  
  #pragma once
  #include "fst/common.h"
  #include "fst/traits.h"
  
  namespace fst {
  template <class T>
  inline constexpr std::conditional_t<!std::is_move_constructible<T>::value && std::is_copy_constructible<T>::value,
      const T&, T&&>
  maybe_move(T& arg) noexcept {
    return std::move(arg);
  }
  
  // Fold expressions for c++ < 17.
  // Calls your function with each of the provided variadic argument.
  template <class Func, class... Args>
  constexpr void fold(Func&& func, Args&&... args) {
    (func(args), ...);
  }
  
  namespace static_for_detail {
    template <class Func, size_t... I>
    constexpr void static_for(Func&& func, std::index_sequence<I...>) {
      return (func(std::integral_constant<size_t, I>{}), ...);
    }
  } // namespace static_for_detail.
  
  // Call a for loop at compile time.
  // Your lambda is provided with an integral_constant.
  // Accept it with auto, access the index with '::value'.
  template <size_t N, class Func>
  constexpr void static_for(Func&& func) {
    static_for_detail::static_for(std::forward<Func>(func), std::make_index_sequence<N>{});
  }
  } // namespace fst.
  