Show that if the left side of the FDE Eq. (29) is zero in a cell, then no change in material content takes place in that cell. Argue that the left side of the FDE Eq. (29) can be made to be zero by either setting c to zero or by setting the difference of the U values at levels j and j + 1 to zero. Thus, one or many FDE cells can be required to have prescribed U-values by setting appropriate entries of the c-array to zero.