code: @Override
  	public boolean equivalent( X x, Y y ) {
  		if ( x == null && y == null )
  			return true;
  		if ( x == null || y == null )
  			return false;
  		return x.equals( y );
  	}
docs: <p>
  {@code equivalent} relies on {@link java.lang.Object#equals(Object) equals}
  to tell whether two objects are equivalent or not. When both arguments are
  {@code null}, it returns {@code true}. When one is {@code null} and the
  other is not {@code null}, it returns {@code false}. Otherwise, it invokes
  {@link java.lang.Object#equals(Object) equals} on the
  <strong>first</strong> object, passing the second object as argument.
  </p>
  
  @param x
  The first object of the comparison.
  @param y
  The second object of the comparison.
  @return {@code true} when the arguments are equivalent. {@code false}
  otherwise.
  @since 1.0