¹It is possible that one may wish to modify the value of the field at the boundary, after a physical boundary condition has already been applied. For example, one may wish to add a small amount of noise at the boundary to test code stability. This added ‘term’ is not a physical boundary condition in itself, however, and this cannot be registered as such. To implement such a scheme one would treat the noise in a manner similar to symmetry boundary conditions, scheduling a routine during BoundaryConditions (see below), after Boundary_ApplyPhysicalBCs, which gets the list of selected variables and adds noise to their boundaries as desired.