HR -- hypertext HR element (horizontal rule)

Description

i1 : DIV{"hi there", HR{}, "ho there" }

o1 = hi there

     ------------------------------------------------------------------------

     ho there

o1 : DIV

i2 : html oo

o2 = <div>
     hi there  <hr/>
     ho there</div>

For the programmer

The object HR is a markup type, with ancestor classes HypertextParagraph < Hypertext < BasicList < Thing.