public Supplier<Token> lookahead() {
        Iterator<Token> iterator = tokens.iterator();
        if (iterator.hasNext()) iterator.next();
        return () -> iterator.hasNext() ? iterator.next() : null;
    }
Returns a sequence of remaining tokens in this, or nulls when none remain.
This may fill the token buffer, but not otherwise modify it.
document/src/main/java/com/yahoo/document/json/TokenBuffer.java