the problem.. while Turing machine's tape is infinite, all real world programs are running within some resource constraints -- there is no such thing as infinite memory. for some programs that ain't a problem -- amount of memory needed by the algoritm can bee known beforehands, at programming time. but for most real world applications memory requirements are not known until run time, and sometimes it is very hard to predict how much does it need. obvious example is an application that allocates memory according to a user input -- for example, an image editor asks user for a dimension of an image he'd like to create. it needs to allocate an array in memory for the image (to have a fast access), so when dimensions exceed possible bounds, good-behaving application should notify user -- and user can either reduce image size or, perhaps, upgrade his machine, if he really wants to work with large images. while it is fairly easy to implement such functionality on a single-task O