Markdown whitespace

by default markdown is used to generate html , and html collapses

several options to change this behavior :
- the html non-breakable space   or <BR>
- the os non-breakable space , on linux : {compse_key} + spac + space (see : [[13. linux compose key]] )
- usa a table
- use a code block
- ? backtick
- use <pre> voor a block

    1 
   / \ 
  2   2 
 / \ / \ 
3  4 4  3