Among much other hacking today, I wrote this code:

       (* val tp_reduce : tp * knd * spine -> tp
           tp_reduce (a, k, sp) computes the result
           of reducing (.\ .\ ... .\ a) . sp
           assuming (.\ .\ ... .\ a) : k
           (where the number of lambdas is the number
            of pis found in k) 
        *) 

	and tp_reduce (a, k, sp) =
	    let 
		fun subst_tpfn s (tpfnLam a) = tpfnLam(subst_tpfn (OneDotShift s) a)
		  | subst_tpfn s (tpfnType a) = tpfnType(subst_tp s a)
		fun tp_reduce'(tpfnLam(a), KPi(_,b,k), h::sp) = 
		    let
			val s = TermDot(termof h, b, Id)
			val a' = subst_tpfn s a
			val k' = subst_knd s k
		    in
			tp_reduce' (a', k', sp)
		    end
		  | tp_reduce' (tpfnType a, Type, []) = a
		  | tp_reduce' _ = raise Syntax "simplified-kind mismatch in type reduction" 
		fun wrap (a, KPi(_,b,k)) = tpfnLam (wrap(a,k))
		  | wrap (a, Type) = tpfnType a
		val aw = wrap (a, k)
	    in 
		tp_reduce' (aw, k, sp)
	    end

to fill a gaping functionality/correctness hole in my typechecker (god damn defined type families with more than one argument!) and it magically typechecked and compiled and passed all tests the first time. That hardly ever happens to me anymore. Very satisfying.