VWMS, chapter 23 (Expressions and constants), page 390

     A  Manifest_string  is  valid  if  and  only  if   it
     satisfies the following two conditions:

        1  None  of  the  characters  of  its   associated
          Simple_string is a double quote.

        2 In the extended form, no characters  other  than
          blanks  or  tabs  may  appear before the initial
          percent sign on the second and subsequent lines.