title: Doxygen does not generate any documentation
body: <p>This is my lexer.hpp file:</p>
  
  <pre>/**
   * \file
   * \brief Lexical analysis
   * \author Igor Zashchelkin
   *
   * Lexical analysis API for MyLisp.
   */
  
  #ifndef LEXER_H
  #define LEXER_H
  
  #include &lt;string&gt;
  #include &lt;vector&gt;
  
  /**
   * \brief Used to link token (Token) with it's type.
   * \date June 29, 2018
   *
   * TokenNumber     - 12, 5.4, -200
   * TokenString     - "hello, world"
   * TokenBoolean    - true, false
   * TokenIdentifier - function name, variable name
   * TokenSeparator  - ( ) ,
   */
  
  enum TokenType {
       TokenNumber /// \brief Tokens which store numeric data
      ,TokenString /// \brief Tokens which store symbolic data, that ends and starts at "
      ,TokenBoolean /// \brief Tokens which store only one state 1 or 0
      ,TokenIdentifier /// \brief Tokens which link to something (variable, function)
      ,TokenSeparator /// \brief Tokens which splits logical parts of code
  };
  
  /**
   * \brief Token's value type
   * \date June 29, 2018
   *
   * Simply, wrap of std::string
   */
  
  typedef std::string TokenValue;
  
  /**
   * \brief Minimal part of lexical analysis
   * \date June 29, 2018
   *
   * Structured pair of TokenType and TokenValue (aka std::string)
   */
  
  class Token {
  private:
      const TokenType type; /// \brief Token's type
      const TokenValue value; /// \brief Token's value
  public:
      Token(TokenType type, std::string value); /// \brief Constructor
  
      const TokenType getType(); /// /brief Getter for type property
      const TokenValue getValue(); /// \brief Getter for value property
  };
  
  /**
   * \brief Lexical analysis API instance
   * \date June 29, 2018
   */
  
  class Lexer {
  private:
      std::string code; /// \brief Source code
  public:
      Lexer(std::string code); /// \brief Constructor
  
      /**
       * \brief Tokenize source code
       * \date June 29, 2018
       *
       * Generate sequence of tokens (std::vector) from code property (std::string)
       */
  
      std::vector tokenize();
  };
  
  #endif //LEXER_H
  </pre>
  
  <p>Everything is OK (that's my mind), but when I run <pre>doxygen src/lexer/lexer.hpp</pre>
  Doxygen generates empty documentation.
  What's wrong?</p>