def VarKeys(constr):
    """Finds the keys in a constraint that represent parameters
    e.g. eliminates any that start with '_'

    :param dict constr: a single constraint entry of form::

        {'var1': mult1, 'var2': mult2,... '_notVar': val,...}

        (see :func:`GroupConstraints`)
    :returns: a list of keys where any keys beginning with '_' are
      removed.
    """
    return [i for i in constr.keys() if not i.startswith('_')]