sys := load Sys Sys->PATH;
millisec: fn(): int;
returns the value of an internal system clock, in milliseconds.
The actual resolution of the clock is a property of the underlying
system and may be coarser than a millisecond.
The epoch is arbitrary, so
is useful for measuring durations but not absolute time.
If the system operates continuously, the millisecond counter overflows
every few months;
a reboot resets the epoch.