'Like ``.var(...)``, except overwrites keys, where .var simply
extends the keys.  Setting a variable to None here will
effectively delete it.'

def setvar(self, **kw):

    kw = self.coerce_vars(kw)
    new_vars = []
    for (name, values) in self.vars:
        if (name in kw):
            continue
        new_vars.append((name, values))
    new_vars.extend(kw.items())
    return self.__class__(self.url, vars=new_vars, attrs=self.attrs, params=self.original_params)
