function Subtract2d( mergedElements ) {

	// Subtract2d inherits from abstract strategy "StableMerge2d".
	
	StableMerge2d.call( this, mergedElements );

	this.strategyType = "Subtract2d";

}
Subtract2d, can be initialized by StrategyFactory directly.
MergeStrategy for MergedLayer when apply Subtract operation to 2d TensorSpace layers.

For example:
```javascript
let conv1d1 = new TSP.layers.Conv1d( { ...config } );
let conv1d2 = new TSP.layers.Conv1d( { ...config } );
let subtractLayer = TSP.layers.Subtract( [ conv1d1, conv1d2 ], { ...config } );
```
In this example, the merged layer "subtractLayer" apply merge strategy "Subtract2d".

@param mergedElements, array of TensorSpace layers. (layerList.length > 0)
@constructor