code: def add(self, parent, child):
          
          if parent not in self:
              if parent.stype != "boolean":
                  raise ValueError("Parent sensor %r is not boolean" % child)
              self._parent_to_not_ok[parent] = set()
          if child not in self:
              if child.stype != "boolean":
                  raise ValueError("Child sensor %r is not booelan" % child)
              self._parent_to_not_ok[child] = set()
          self.add_links(parent, (child,))
docs: Add a pair of boolean sensors.
  
          Parent depends on child.
  
          Parameters
          ----------
          parent : boolean instance of :class:`katcp.Sensor`
              The sensor that depends on child.
          child : boolean instance of :class:`katcp.Sensor`
              The sensor parent depends on.