def add(self, elem, replace=False):
        """Add a requirement.

        :param elem: a string or :class:`Requirement` instance.
        :param replace: if `True`, packages in the set with the same name will
            be removed first.
        """
        if isinstance(elem, text_type):
            elem = Requirement.coerce(elem)
        if replace and elem in self:
            self.remove(elem)
        super(Requirements, self).add(elem)
def __init__(self):
        # Fix some key bindings
        self.bind("<Control-Key-a>", self.select_all)
        # We will need Ctrl-/ for the "stroke", but it cannot be unbound, so
        # let's prevent it from being passed to the standard handler
        self.bind("<Control-Key-/>", lambda event: "break")
        # Diacritical bindings
        for a, k in self.accents:
            # Little-known feature of Tk, it allows to bind an event to
            # multiple keystrokes
            self.bind("<Control-Key-%s><Key>" % k,
                      lambda event, a=a: self.insert_accented(event.char, a))