User Defined tags - Nested loops - syntax

[olavt]

When I try to put a nested loop in a user defined tag, I get the red "invalid code" warning, no matter what I try to do. And I do not find any information on a specific syntax for this. This is an abbrevated version of the string:

while ($row = mysql_fetch_array($books)) {

echo 'string

if($row['SMPlus']!=NULL){echo '
  SheetmusicPlus';}

string contiunes';
}

I have tried to use and not use ' at the points where the string breaks and the inner condition start and when the string continues. I have tried with $row['SMPlus'] and .$row['SMPlus']. in the condition, with and without semicolon at the end of the inner condition, but nothing works. Where do I find information on how to do this?