Laravel migration foreign keys references to a primary key with two columns?

I am using Laravel 5.3 and MySQL.

How can I add in laravel foreign keys references to a primary key with two columns?

I have something like

primary key with two columns

public function up()
    {
        Schema::create('X', function (Blueprint $table) {
            $table->integer('a')->unsigned();
            $table->integer('b')->unsigned();
            $table->primary(['a', 'b']);
            $table->timestamps();
        });
    }

and in another

public function up()
    {
        Schema::create('Y', function (Blueprint $table) {
            $table->increments('k');
            $table->foreign('c')->references(['a', 'b'])->on('X')->onDelete('cascade');
            $table->timestamps();
        });
    }

However, it doesn't work so what should I put?

Any help would be really appreciated, Thanks in advance

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