code: public static ImmutableMatrix identity(final int size) {
  		if (size <= 0) throw new IllegalArgumentException("Invalid size");
  		return create(new ImmutableData(size, size) {
  			@Override
  			public double getQuick(int row, int col) {
  				return row == col ? 1 : 0;
  			}
  		});
  	}
docs: Returns an immutable identity matrix of the specified dimension.
  
  @param size the dimension of the matrix
  @return a <code>size</code>×<code>size</code> identity matrix
  @throws IllegalArgumentException if the size is not positive