#####
##### Boundary conditions on pressure are derived from boundary conditions
##### on the north-south horizontal velocity, v.
#####

# Pressure boundary conditions are either zero flux (Neumann) or Periodic.
PressureBC(::BC)  = BoundaryCondition(Flux, nothing)
PressureBC(::PBC) = PeriodicBC()

function PressureBoundaryConditions(vbcs)
    x = CoordinateBoundaryConditions(PressureBC(vbcs.x.left), PressureBC(vbcs.x.right))
    y = CoordinateBoundaryConditions(PressureBC(vbcs.y.left), PressureBC(vbcs.y.right))
    z = CoordinateBoundaryConditions(PressureBC(vbcs.z.left), PressureBC(vbcs.z.right))
    return (x=x, y=y, z=z)
end
