public void add(Key k, Value v) throws IllegalArgumentException {
    add(k, v.get());
  }
public void add ( Key k , Value v ) throws IllegalArgumentException { add ( k , v . get ( ) ) ; }
When empty, the first key added sets the row+column. After this all keys added must have the
same row+column.

@param k Key to be added to buffer
@param v Value to be added to buffer