code: public static Schema enumWith(Iterable<String> values) {
      Set<String> uniqueValues = ImmutableSet.copyOf(values);
      Preconditions.checkArgument(uniqueValues.size() > 0, "No enum value provided.");
      Preconditions.checkArgument(Iterables.size(values) == uniqueValues.size(), "Duplicate enum value is not allowed.");
      return new Schema(Type.ENUM, uniqueValues, null, null, null, null, null, null);
    }
docs: Creates a {@link Schema} of {@link Type#ENUM ENUM} type, with the given enum values.
  The set of values given should be unique and must contains at least one value.
  The ordering of values in the enum type schema would be the same as the {@link Iterable#iterator()} order.
  
  @param values Enum values.
  @return A {@link Schema} of {@link Type#ENUM ENUM} type.