Does ApproxFun currently have a way to solve BVPs with mass conservation? For example 0=u''+f(u,v) and 0=v''-f(u,v) with homogeneous Neumann boundary conditions or periodic boundary conditions. Namely, can I tell ApproxFun what the total mass is so that the linearization of this operator is invertible? Something like \int u+vdx=A.