https://github.com/SciML/SciMLBase.jl/pull/306/files#diff-e6e23faf8a4ea0af48109781a6a7a90ed8fd2f3afa010176f37d2e37b5c98190R12

0 = f(u_{n+1}, u_{n}, p, t_{n+1}, integ)

we want a system type that does this as a nice building block for a lot of things, such as new advanced DAE solvers. @ErikQQY's work finished DiscreteSystem (#2058) so now I think we're ready to do it's "final boss". I wonder if pieces from DiscreteSystem can be reused, though I think this will need to be its own system type.