// return whether there are some elements that are fix inline bool has_no_fixed_elements() const { return fix.empty(); }
// return whether the list contains no element bool empty() const{ return fix.empty() && variable.empty(); }
// return whether there are some elements that are fix inline bool has_no_fixed_elements() const { return fix.empty(); }
// return whether the list contains no element bool empty() const{ return fix.empty() && variable.empty(); }