'Substitutes the entry under ``vspec.name`` with ``vspec``.
Args:
vspec: variant spec to be substituted'

def substitute(self, vspec):

    if (vspec.name not in self):
        msg = 'cannot    substitute    a    key    that    does    not    exist    [{0}]'
        raise KeyError(msg.format(vspec.name))
    super(VariantMap, self).__setitem__(vspec.name, vspec)
