code: func (c *Corpus) str(s string) string {
  	if s == "" {
  		return ""
  	}
  	if s, ok := c.strs[s]; ok {
  		return s
  	}
  	if c.strs == nil {
  		c.strs = make(map[string]string)
  	}
  	c.strs[s] = s
  	return s
  }
docs: // str returns s, interned.