=begin filter html nnn=1

line 1

line 2

line 4

=end filter __RESULT__ +html
  1 <p>line 1</p>
  2     <p>line 2</p>
  3 
  4     <p>line 4</p>
__RESULT__ -html
<p>line 1</p>
    <p>line 2</p>

    <p>line 4</p>