private static Principal getPrimaryPrincipal(final PrincipalElectionStrategy principalElectionStrategy,
                                                 final Set<Authentication> authentications,
                                                 final Map<String, List<Object>> principalAttributes) throws Throwable {
        return principalElectionStrategy.nominate(new LinkedHashSet<>(authentications), principalAttributes);
    }
Principal id is and must be enforced to be the same for all authentications.
Based on that restriction, it's safe to grab the first principal id in the chain
when composing the authentication chain for the caller.
core/cas-server-core-authentication-api/src/main/java/org/apereo/cas/authentication/DefaultAuthenticationResultBuilder.java