def html_id(prefix, index)
      prefix = prefix || self.engine_id
      prefix, index = prefix.to_s, index.to_s

      return nil if index.empty?
      return nil if prefix.empty?

      return "#{prefix}_#{index}"
    end
def html_id(prefix, index)
      prefix = prefix || self.engine_id
      prefix, index = prefix.to_s, index.to_s

      return nil if index.empty?
      return nil if prefix.empty?

      return "#{prefix}_#{index}"
    end
Can be used as an id attribute for anchor destination in HTML.
 Will return "#{prefix}_#{index}" -- if prefix is missing,
 will use #engine_id if present. If both are missing, returns nil.
 if index missing, returns nil.