code: def get_common_complete_suffix(document, completions):
      
      # Take only completions that don't change the text before the cursor.
      def doesnt_change_before_cursor(completion):
          end = completion.text[:-completion.start_position]
          return document.text_before_cursor.endswith(end)
  
      completions2 = [c for c in completions if doesnt_change_before_cursor(c)]
  
      # When there is at least one completion that changes the text before the
      # cursor, don't return any common part.
      if len(completions2) != len(completions):
          return ''
  
      # Return the common prefix.
      def get_suffix(completion):
          return completion.text[-completion.start_position:]
  
      return _commonprefix([get_suffix(c) for c in completions2])
docs: Return the common prefix for all completions.