You may want a more intuitive dictionary key. For one, the `l` parameter is arbitrary, since it is the index of the function in `func`, but there is nothing specifically that defines the ordering of the functions or makes an ordering special. Maybe something like `func_coeff[eq_number, function, diff_order]`? (where `eq_number` would be `j`, `function` would be `func[l]`, and `diff_order` would be `k`)