Robinson's unification algorithm required time exponential in the size
of terms. The following years saw a sequence of unification algorithms cul-
minating with the linear-time algorithm of Paterson and Wegman [1978].
Unification can also be generalized to higher-order logics and equational
theories (Huet and Oppen [1980]). These generalizations, although quite inter-
esting, usually lead to hard computational questions outside PTIME. We con-
centrate on parallel (first-order uninterpreted) unification, and we review the
lower and upper bounds appearing in Dwork et al. [1984, 1986 ...
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