code: public static Expression in(String lhs, Object... rhs) {
          Expression ex = new Expression(lhs, "$in", rhs);
          if (rhs.length == 1) {
              ex.single = true;
          }
          return ex;
      }
docs: The document field must exist in the list provided
  @param lhs The field to check
  @param rhs The argument - one or more values
  @return Expression: lhs $in rhs