code: def include_once(filename, lineno, local_first):
      
      filename = search_filename(filename, lineno, local_first)
      if filename not in INCLUDED.keys():  # If not already included
          return include_file(filename, lineno, local_first)  # include it and return
  
      # Now checks if the file has been included more than once
      if len(INCLUDED[filename]) > 1:
          warning(lineno, "file '%s' already included more than once, in file "
                          "'%s' at line %i" %
                  (filename, INCLUDED[filename][0][0], INCLUDED[filename][0][1]))
  
      # Empty file (already included)
      LEXER.next_token = '_ENDFILE_'
      return ''
docs: Performs a file inclusion (#include) in the preprocessor.
      Writes down that "filename" was included at the current file,
      at line <lineno>.
  
      The file is ignored if it was previuosly included (a warning will
      be emitted though).
  
      If local_first is True, then it will first search the file in the
      local path before looking for it in the include path chain.
      This is used when doing a #include "filename".