KMain()
    includes ::/StartOS.ZC
        includes ~/MakeHome.ZC
            includes ~/HomeSys.ZC
                calls StartUpTasks()
                    includes ~/Once.ZC
                        calls OnceExe().

SysOnce() appends src code to ~/Registry.ZC Once/System tree, executed at next boot by System.

Once()  appends src code to ~/Registry.ZC Once/User tree, executed at next boot by first User term.

At boot, OnceExe(), executes Once/System tree, SysOnceFlush()s it,
executes Once/User tree and OnceFlush()s.