My Problem: I'm using the a21_glossary extension. It has everything i need, except
that the glossary page lists all entries in a row without dividing between letters


NATO
  Description text
UNO
  Description text
WWF
  Description text


What i want is

N
NATO
  Description text
U
UNO
  Description text
W
WWF
  Description text


So what i did was to modify the extension class. In this class there is a loop through all glossar entries 
In every loop i pick the first letter and compare if it has changed compared to the last loop. Then i:

if ($currentFirstLetter != $lastFirstLetter) {
  $row['first_letter'] = $currentFirstLetter;
} else {
  unset($row['first_letter']);
}

Shortly after that the $row array is assigned to the cObj:

$cObj->data = $row;

So far everythings fine. I would expect the first_letter var to be accessible in the typoscript section, which is:

tx_a21glossary_main = COA
tx_a21glossary_main {

   10 = TEXT
   10.if.isTrue.field = first_letter
   10.field = first_letter
   10.wrap = <h2>|</h2>

   20 = TEXT 
   20.if.isTrue.field = short
   20.field = short
   20.wrap = {$a21glossary.shortWrap}

   30 = COA
   30.wrap = {$a21glossary.restWrap}
   30 {
      10 = TEXT
      10.if.isTrue.field = description
      10.field = description
      10.wrap = {$a21glossary.descriptionWrap}
   }
}

But it seems the "first_letter" field is not the field of the array handed over to the cObj, but rather the database column, since the "if.isTrue" doesn't work for "first_letter"...