Variable "<str>" is a FOR loop counter - assignment not allowed

  Explanation:  The named variable was used by the FOR counted
  loop which makes it a CONSTANT for the duration of the loop
  body.  This variable is the target of a SET, INSERT...RETURNING,
  SELECT...INTO, UPDATE...RETURNING, or GET GIAGNOSTICS statement
  or CALL output parameter.

  User Action:  Check that the correct variable is used in this
  statement.  If you need to update the loop counter, replace the
  FOR IN loop with a WHILE loop.