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    # unless object is valid and either isn't busy or can wait:
    unless(  $a  &&  ref($a)  &&  isa($a,'Foo')
         &&  (  ! $a->IsBusy()  ||  $a->CanWait()  )  ) {
        return;
    }

    # if object is not valid or both is busy and can't wait:
    if(  ! $a  ||  ! ref($a)  ||  ! isa($a,'Foo')
     ||  $a->IsBusy()  &&  ! $a->CanWait()  ) {
        return;
    }