This function returns information about one SGML Declaration feature for the document type associated with doc, or the current document if omitted. feature is one of the strings subdoc, formal, concur, explicit, implicit, simple, rank, datatag, omittag, or shorttag. The return value is 1, 0, or -1. Some features may be 1 (on) or 0 (off) and others are 0 or a positive number. This command returns -1 if feature is invalid. The case of the string is not important.