If there is an element after the last element of the sub - matrix then decrement it by 1
if ( Y2 + 1 < n ) : NEW_LINE INDENT mat [ X2 ] [ Y2 + 1 ] = mat [ X2 ] [ Y2 + 1 ] - 1 ; NEW_LINE DEDENT elif ( X2 + 1 < n ) : NEW_LINE INDENT mat [ X2 + 1 ] [ 0 ] = mat [ X2 + 1 ] [ 0 ] - 1 ; NEW_LINE DEDENT