The difference is actually explained. The `seq-` functions are generic. From the same document I linked to before: > The seq.el library provides the following

The difference is actually explained. The `seq-` functions are generic. From the same document I linked to before: > The seq.el library provides the following additional sequence manipulation macros and functions, prefixed with seq-. To use them, you must first load the seq library. > All functions defined in this library are free of side-effects; i.e., they do not modify any sequence (list, vector, or string) that you pass as an argument. Unless otherwise stated, the result is a sequence of the same type as the input. For those functions that take a predicate, this should be a function of one argument. > The seq.el library can be extended to work with additional types of sequential data-structures. For that purpose, all functions are defined using cl-defgeneric. See Generic Functions, for more details about using cl-defgeneric for adding extensions. That statement occurs before the various `seq-` functions are listed and described so it's hard for me understand the claim that "it's not always clear what the difference is" when it's stated so clearly before the functions themselves. And when you get into them, like with `seq-length`, you get a statement like this: > For built-in sequence types, seq-length behaves like length. I really don't see how they could present this more clearly, what's your suggestion? Should they put flashing lights around the text?