Each cell in the spreadsheet holds a numerical value, a formula, or a (possibly empty) plain text. As mentioned at the beginning of the chapter, a formula is a text beginning with the equal sign (=) followed by a numerical expression with cell references. If the cell holds a value, it may affect the values in other cells (if it does not hold a value, it might cause evaluation errors in target cells). If the cell contains a formula, its value may depend on the values in other cells. This implies that each cell needs a set of cells that it depends on, that is, its source set, and a set of cells that depend on it, that is, its target set.

Only a formula has a non-empty source set, which is the set of all references of the formula. The target set, on the other hand, is more complicated–a cell does not decide its own target set; it is decided indirectly by the formulas that have it as its source cell.

In mathematical terms, the cells with its source and target sets constitute...