The subroutine bond applies the correct boundary conditions between regions of different conductivity by choosing the correct value for the conductivity of a bond that extends across a boundary. Remember that in the finite difference programs, the node is thought of as being at the center of a pixel, with nearest neighbor bonds connecting nodes. A bond that connects nodes that are in regions of different conductivities gets assigned a conductance that is a function of the voxel geometry and is a series combination of the two different values of conductivities. When a bond connects two nodes of the same conductivity, then that value of the conductivity is simply used to assign the conductance of the bond.