code: func (d *Database) PlaceholdersString(src interface{}, includePk bool) (string, error) {
  	lst, err := d.Placeholders(src, includePk)
  	if err != nil {
  		return "", err
  	}
  	return strings.Join(lst, ","), nil
  }
docs: // PlaceholdersString returns a list of placeholders suitable for an INSERT
  // or UPDATE query in string form, e.g.:
  //   ?,?,?,?
  // if includePk is false, the primary key field is omitted.