Systems and methods, including computer software products, can be used to update or modify data stored in a memory. One or more variables are represented with one or more cell values in a memory. Each variable is associated with one or more of the cell values. Multiple states of the one or more variables are defined, and each defined state of the one or more variables includes a current store value for each variable and at least one previous store value for the variable. One or more single cell values influence the current store value and previous store value of at least one variable.