'Add a new item to the minimal set, having the specified
context, target, and display form.
:param context: The context in which the item of interest appears
:type context: str
:param target: The item of interest
:type target: str
:param display: The information to be reported for each item
:type display: str'

def add(self, context, target, display):

    self._seen[context].add(target)
    self._contexts.add(context)
    self._targets.add(target)
    self._displays[(context, target)] = display
