Caveat: I haven't used Fennel but just inferring from what makes sense... The Lua VM that they compile to already handles missing arguments with some attendant

Caveat: I haven't used Fennel but just inferring from what makes sense... The Lua VM that they compile to already handles missing arguments with some attendant but unavoidable performance cost. Fennel's own `lambda` form that they layer on top does check for missing arguments, but it must do so by generating extra Lua code to do those checks. That additional code has a runtime cost. Using `fn` avoids that extra generated code and its cost. If Fennel had its own runtime and VM, then the performance story for how missing arguments are handled would be different.