@@ -95,6 +95,11 @@ public final class Tuple0 implements Tuple, Comparable<Tuple0>, Serializable {
         return List.empty();
     }
 
+    public <T1> Tuple1<T1> concat(T1 t1) {
+        Objects.requireNonNull(t1, "t1 is null");
+        return Tuple.of(t1);
+    }
+
     // -- Object
 
     @Override
/*  __    __  __  __    __  ___
 * \  \  /  /    \  \  /  /  __/
 *  \  \/  /  /\  \  \/  /  /
 *   \____/__/  \__\____/__/
 *
 * Copyright 2014-2018 Vavr, http://vavr.io
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package io.vavr;

/*-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-*\
   G E N E R A T O R   C R A F T E D
\*-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-*/

import io.vavr.collection.Iterator;
import io.vavr.collection.List;
import io.vavr.collection.Seq;
import java.io.Serializable;
import java.util.Comparator;
import java.util.Objects;
import java.util.function.Supplier;

/**
 * A tuple of no elements which can be seen as cartesian product of no components.
 *
 * @author Daniel Dietrich
 */
public final class Tuple0 implements Tuple, Comparable<Tuple0>, Serializable {

    private static final long serialVersionUID = 1L;

    /**
     * The singleton instance of Tuple0.
     */
    private static final Tuple0 INSTANCE = new Tuple0 ();

    /**
     * The singleton Tuple0 comparator.
     */
    private static final Comparator<Tuple0> COMPARATOR = (Comparator<Tuple0> & Serializable) (t1, t2) -> 0;

    // hidden constructor, internally called
    private Tuple0 () {
    }

    /**
     * Returns the singleton instance of Tuple0.
     *
     * @return The singleton instance of Tuple0.
     */
    public static Tuple0 instance() {
        return INSTANCE;
    }

    public static  Comparator<Tuple0> comparator() {
        return COMPARATOR;
    }

    @Override
    public int arity() {
        return 0;
    }

    @Override
    public int compareTo(Tuple0 that) {
        return 0;
    }

    /**
     * Transforms this tuple to an object of type U.
     *
     * @param f Transformation which creates a new object of type U based on this tuple's contents.
     * @param <U> type of the transformation result
     * @return An object of type U
     * @throws NullPointerException if {@code f} is null
     */
    public <U> U apply(Supplier<? extends U> f) {
        Objects.requireNonNull(f, "f is null");
        return f.get();
    }

    @Override
    public Seq<?> toSeq() {
        return List.empty();
    }

    // -- Object

    @Override
    public boolean equals(Object o) {
        return o == this;
    }

    @Override
    public int hashCode() {
        return 1;
    }

    @Override
    public String toString() {
        return "()";
    }

    // -- Serializable implementation

    /**
     * Instance control for object serialization.
     *
     * @return The singleton instance of Tuple0.
     * @see java.io.Serializable
     */
    private Object readResolve() {
        return INSTANCE;
    }

}
Do we need this check here?