Sig_Val value: This is an enumerated type which
contains the value of the signal. This value can be one of
SIG_HIGH, SIG_LOW, SIG_X or
SIG_NULL. The latter signal value represents an invalid
signal.
ckt_time t: This data member represents the time at which
the signal occurred. The ckt_time type is defined to be of type
long in this particular implementation. Two constants,
CKT_TIME_INIT and CKT_TIME_NULL represent an
initial time and an invalid time respectively.