using System;

public interface ISequenced[T]
{
   Equals (mutable  that : ISequenced[T]) : bool;
}

public class SequencedHasher [S,W]
  where S : class, ISequenced[W]
{
        public Equals (mutable  i1 : S,mutable  i2 :  S) :  bool
  {
     if ( i1 == null) i2 == null else  i1.Equals (i2);
  }
}

public class Sequenced[T] : ISequenced[T]
{
  public Equals (mutable  _that : ISequenced[T]) :  bool
  {
     false;
  }
}

class X
{
  static Main () :  void
  {
    mutable  s =  Sequenced ();
    mutable  hasher =  SequencedHasher ();
    assert (!hasher.Equals (s, s));
  }
}
/*
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*/