 By adding additional uncertainty to the initial market model, via this random time, those structures may fail{\it   .
<meaning-changed>  By adding additional uncertainty to the initial market model, via this random time, those structures may fail{\it   .
 By adding additional uncertainty to the initial market model, via this random time, those structures may fail{\it may fail and hence the Markowitz's optimal portfolio and other quadratic-optimal portfolios might fail to exist .