Reverse Turing test if the subject is or is not human. As of 2004, such procedures, called CAPTCHAs, are used in some anti-spam systems to prevent automated bulk use of communications systems. The use of captchas is controversial. Circumvention methods exist that reduce their effectiveness. Also, many implementations of captchas (particularly ones desired to counter circumvention) are inaccessible to humans with disabilities, and/or are difficult for humans to pass. Note that "CAPTCHA" is an acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart" so that the original designers of the test regard the test as a Turing test to
Turing machine for which the action table has at most one entry for each combination of symbol and state. Read-only, right-moving Turing machines are equivalent to NDFAs (as well as DFAs by conversion using the NDFA to DFA conversion algorithm). For practical and didactical intentions the equivalent register machine can be used as a usual assembly programming language. An interesting question is whether the computation model represented by concrete programming languages is Turing equivalent. While the computation of a real computer is based on finite states and thus not capable to simulate a Turing machine, programming languages themselves do not necessarily have