Femtolisp looks interesting, although the weak documentation makes it hard to know. It it has no lexical scoping, no TCO, and no hygenic macros (I don't care if it's lowlevel or highlevel (er-macro-expander, or syntax-rules), or even if it's fexprs like Kernel, but it has to be hygenic), I tend to dismiss it out of hand, unless it has some really cool feature that means it can't, or is otherwise interesting (newlisp, picolisp). But femtolisp has most of that, so my only concerns are lack of hygene (which is a big concern in a lisp-1), and taking too much of the large and unweildly r6rs. But that would be hard, given the size.