'Returns a column matrix containing all of the symbols in the system
that depend on time'

def dynamic_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[:]
    functions_of_time = set()
    for expr in eom_expressions:
        functions_of_time = functions_of_time.union(find_dynamicsymbols(expr))
    functions_of_time = functions_of_time.union(self._states)
    return tuple(functions_of_time)
