Ashok Chandra, Dexter Kozen, and Larry
Stockmeyer, <a href="alternation.pdf">"Alternation,"</a>
    <i> JACM,</i>  28(1), (1981), 114-133.<p>

This paper introduces the notion of alternation for
Turing machines.  This is a robust and useful generalization of nondeterminism.