A call to register_trace(n, r, m) registers the integer reference
variable r as a tracing variable associated with name n. The
integer m is its maximum value. Its value at the time of
registration is considered its default value, which will be restored
by a call to reset_trace n or reset_traces.
FAILURE
Fails if there is already a tracing variable registered under the name
given, or if either the maximum value or the value in the reference is
less than zero.