Many properties of semisimple Lie algebras depend only on reducibility. An ongoing area of research in recursion theory studies reducibility relations other than Turing reducibility. The most well known are arithmetical reducibility and hyperarithmetical reducibility. See Weyl's theorem on complete reducibility. Both axioms, however, were met with skepticism and resistance; see more at Axiom of reducibility. The most fundamental reducibility notion is Turing reducibility. This observation plays an important role in the proof of Weyl's theorem on complete reducibility. Enumeration operators are of central importance in the study of enumeration reducibility. There are two common ways of producing reductions stronger than Turing reducibility. Type theory is chiefly interested in the convertibility or reducibility of programs. The correctness of this algorithm relies on a property of its distance function called "reducibility". It does not matter whether many-one reducibility or Turing reducibility is used. In 1944 Emil Post used the term "Turing reducibility" to refer to the concept. One method is based on the difference in the reducibility between the two tetrahalides. A technical detail not discussed here but required to complete the proof is "immersion reducibility".