code: private SingleType parseSingleType() throws TTXPathException {
  
          final String atomicType = parseAtomicType();
          final boolean intero = is(TokenType.INTERROGATION, true);
          return new SingleType(atomicType, intero);
      }
docs: Parses the the rule SingleType according to the following production
  rule:
  <p>
  [48] SingleType ::= AtomicType "?"? .
  </p>
  
  @return SingleType
  @throws TTXPathException