'Clear the value attribute of a control.
As a result, the affected control will not be successful until a value
is subsequently set.  AttributeError is raised on readonly controls.'

def clear(self, name=None, type=None, kind=None, id=None, nr=None, label=None):

    c = self.find_control(name, type, kind, id, label=label, nr=nr)
    c.clear()
