 For instance , when one cell of a spreadsheet changes , it is necessary to <unk> the values of other cells that depend directly or indirectly on the changed cell . For this problem , the tasks to be scheduled are the <unk> of the values of individual cells of the spreadsheet . Dependencies arise when an expression in one cell uses a value from another cell . In such a case , the value that is used must be <unk> earlier than the expression that uses it . <unk> ordering the dependency graph , and using this topological order to schedule the cell updates , allows the whole spreadsheet to be updated with only a single evaluation per cell . Similar problems of task ordering arise in <unk> for program compilation and instruction scheduling for low @-@ level computer program <unk> . 
