'Returns a column matrix containing all of the symbols in the system
that do not depend on time'

def constant_symbols(self):

    if (self._comb_explicit_rhs is None):
        eom_expressions = (self.comb_implicit_mat[:] + self.comb_implicit_rhs[:])
    else:
        eom_expressions = self._comb_explicit_rhs[:]
    constants = set()
    for expr in eom_expressions:
        constants = constants.union(expr.free_symbols)
    constants.remove(dynamicsymbols._t)
    return tuple(constants)
