public ValueType value(Class<?> type) {
    final List<ValueType> values = values(type);
    if (values.size() != 1) {
      throw new IllegalArgumentException(String.format("Expected 1 values but got %d for %s", values.size(), type));
    }
    return values.get(0);
  }
Return single {@link ValueType} instance associated with immutable class {@code type}.
@throws IllegalArgumentException if multiple {@link ValueType}s are associated with {@code type}
processor-testlib/src/org/immutables/value/processor/meta/ProcessorRule.java