docs: Expression XreadValueSpecificationOrNull() {
  
          Expression e     = null;
          boolean    minus = false;
  
          switch (token.tokenType) {
  
              case Tokens.PLUS :
                  read();
                  break;
  
              case Tokens.MINUS :
                  read();
  
                  minus = true;
                  break;
          }
  
          e = XreadUnsignedValueSpecificationOrNull();
  
          if (e == null) {
              return null;
          }
  
          if (minus) {
              e = new ExpressionArithmetic(OpTypes.NEGATE, e);
          }
  
          return e;
      }