Windable Heads & Recognizing NL with Constant Randomness

Every language in NL has a \(k\)-head two-way nondeterministic finite automaton (2nfa(\(k\))) recognizing it. It is known how to build a constant-space verifier algorithm from a 2nfa(\(k\)) for the same language with constant-randomness, but with error probability \(\tfrac{k^2-1}{2k^2}\) that can not be reduced further by repetition. We have defined the unpleasant characteristic of the heads that causes the high error as the property of being "windable". With a tweak on the previous verification algorithm, the error is improved to \(\tfrac{k_{\textrm{W}}^2-1}{2k_{\textrm{W}}^2}\), where \(k_{\textrm{W}} \le k\) is the number of windable heads. Using this new algorithm, a subset of languages in NL that have a 2nfa(\(k\)) recognizer with \(k_{\textrm{W}} \le 1\) can be verified with arbitrarily reducible error using constant space and randomness.