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diff --git a/libbutl/string-table.mxx b/libbutl/string-table.mxx
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+// file      : libbutl/string-table.mxx -*- C++ -*-
+// copyright : Copyright (c) 2014-2017 Code Synthesis Ltd
+// license   : MIT; see accompanying LICENSE file
+
+#ifndef __cpp_modules
+#pragma once
+#endif
+
+#include <cassert>
+
+#ifndef __cpp_lib_modules
+#include <string>
+#include <vector>
+#include <unordered_map>
+
+#include <limits>  // numeric_limits
+#include <cstddef> // size_t
+#endif
+
+// Other includes.
+
+#ifdef __cpp_modules
+export module butl.string_table;
+#ifdef __cpp_lib_modules
+import std.core;
+#endif
+import butl.multi_index;
+#else
+#include <libbutl/multi-index.mxx>
+#endif
+
+#include <libbutl/export.hxx>
+
+LIBBUTL_MODEXPORT namespace butl
+{
+  // A pool of strings and, optionally, other accompanying data in which each
+  // entry is assigned an individual index (or id) of type I (e.g., uint8_t,
+  // uint16_t, etc., depending on how many entries are expected).  Index value
+  // 0 is reserved to indicate the "no entry" condition.
+  //
+  template <typename I, typename D>
+  struct string_table_element
+  {
+    const I i;
+    const D d;
+  };
+
+  template <typename I>
+  struct string_table_element<I, std::string>
+  {
+    const I i;
+    const std::string d;
+  };
+
+  // For custom data the options are to call the data member 'key' or to
+  // specialize this traits.
+  //
+  template <typename D>
+  struct string_table_traits
+  {
+    static const std::string&
+    key (const D& d) {return d.key;}
+  };
+
+  template <>
+  struct string_table_traits<std::string>
+  {
+    static const std::string&
+    key (const std::string& d) {return d;}
+  };
+
+  template <typename I, typename D = std::string>
+  struct string_table
+  {
+    // Insert new entry unless one already exists.
+    //
+    I
+    insert (const D&);
+
+    // Find existing.
+    //
+    I
+    find (const std::string& k) const
+    {
+      auto i (map_.find (key_type (&k)));
+      return i != map_.end () ? i->second.i : 0;
+    }
+
+    // Reverse lookup.
+    //
+    const D&
+    operator[] (I i) const {assert (i > 0); return vec_[i - 1]->second.d;}
+
+    I
+    size () const {return static_cast<I> (vec_.size ());}
+
+    bool
+    empty () const {return vec_.empty ();}
+
+    void
+    clear () {vec_.clear (); map_.clear ();}
+
+  private:
+    using key_type = butl::map_key<std::string>;
+    using value_type = string_table_element<I, D>;
+    using map_type = std::unordered_map<key_type, value_type>;
+    using traits = string_table_traits<D>;
+
+    map_type map_;
+    std::vector<typename map_type::const_iterator> vec_;
+  };
+}
+
+#include <libbutl/string-table.txx>