public boolean isPrimary() {
      return isPrimary;
    }
Guaranteed to be true in exactly one entry.

<p>Note: currently this may be false for all entries, since it is possible that keysets are
parsed without a primary. In the future, such keysets will be rejected when the keyset is
parsed.
java_src/src/main/java/com/google/crypto/tink/KeysetHandle.java