clock                                              Sets the period and
                                                   boundary conditions of a
                                                   clock.
 signal_name                                         an input port or any wire
                                                     at current scope
 -rise                      rise                     >= 0, < period, != fall
 -fall                      fall                     >= 0, < period, != rise
 -period                    period
 [ -minusskew               minus_skew ]
 [ -plusskew                plus_skew ]
 [ -multiplier              multiplier ]             must be greater than 0/1
