Is it possible to evaluate a math expression in XSLT. GIven a
  variable $pos, I would like to compute the value of

  ($pos+1) * $max-len and use it in an attribute thus

  <td colspan="{expr}"> where expr is the result of evaluating above
  expression

Did you try this? (it would have been quicker than mailing the list)
You just do exactly what you said:

 <td colspan="{($pos+1) * $max-len}">

David

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