private static Set<TokenType> getKeywordSet(final TokenType tokenType, final Map<TokenType, Set<TokenType>> map) {
        if (tokenType != null) {
            final Set<TokenType> set = map.get(tokenType);
            if (set != null) {
                return set;
            }
        }
        return Collections.emptySet();
    }
@return A set of {@link TokenType}s that are valid to appear in the query after tokenType.
stroom-query/stroom-query-api/src/main/java/stroom/query/api/token/TokenType.java