private void validate() {
            // XXX not fool proof length test, should test codepoint by codepoint for mixed case user input? not even that will necessarily be 100% robust...
            String asLow = toLowerCase(token);
            // TODO: Put along with the global toLowerCase
            String asHigh = token.toUpperCase(Locale.ENGLISH);
            if (asLow.length() != token.length() || asHigh.length() != token.length()) {
                throw new IllegalArgumentException("Special token '" + token + "' has case sensitive length. " +
                                                   "Please report this to the Vespa team.");
            }
        }
Returns the token to replace occurrences of this by, which equals token() unless this has a replacement.
linguistics/src/main/java/com/yahoo/language/process/SpecialTokens.java