  We present a design and protocol to add binary numbers using discrete
solitons in waveguide arrays. We show that the nonlinear interaction between
discrete solitons in waveguide arrays can be exploited to design half and full
adders. By modulating the separation between waveguides and introducing control
solitons, we achieve the performance of an XOR gate. We construct the half and
full adders using the XOR gate together with the previously-designed OR and AND
gates. To facilitate the experimental realization, we calculate the profile of
separations between the waveguides that will lead to the performance of the XOR
gate.
