And I. OP with partial-order precedence, mixfix ops, op predicates for white-space sensitivity, and op-fragment chains for n-ary mixfix. RD with backtracking, computed predicate rules, and backtracked "parse scoped" data. The thought of exploring a performance-oriented one in Julia (monomorphized multiple-dispatch jit) occasionally comes to mind - if I ever see a "get the rep right and the problem gets much easier" story for graph rewrite on an LLVM scale, I might try for an fully-extensible compiler bootstrap.