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Isham 5 Decoherence Functional
The key technical ingredient is the decoherence functional D(h',h) which is
a function of pairs of histories h',h associated to the same collection of
questions Q1...QN,,, If

h' = P(QN,q'N,tN)...P(Q1,q'1,t1)   (6.3.6)

[Q is set of all questions pertaining to a particular measurement.  q is
one such question]

...

D(h',h) = tr{P(QN,q'N,tN)...P(Q1,q'1,t1)p(0)P(QN,qN,tN)...P(Q1,q1,t1)}
(6.3.8)

The family of histories is said to be consistent if D(h',h) = 0 for all
pairs h,h' for which h not equal to h'..

6.3.3 The Application to Quantum Gravity
The consistency condition (6.3.9) depends on the state p(0)... As
emphasised by Gell-Mann and Hartle, this implies that the manifest
existence of our current world of a quasi-classical domain depends
ultimately on the initial state p(0) that existed shortly after the
'initial' big bang.  From this perspective, the classical features of our
present-day world must be seen as a contingent property of the big-bang:
they could have been otherwise.

[I propose that the structure of the 'contingent property' is engineered by
advanced intelligence in our far future using controlled quantum action at
a distance backward in time to bring themselves (and us) into being.
Sarfatti]

Indeed, for all we know, the quantum theory of our universe may admit other
consistent families of histories with no quasi-classical domains at all; a
valid concept in the context of a post-Everett interpretation of quantum
theory." p.100

[The other consistent families are like the Q-space of super-beings in Star
Trek - nonlocal discarnate intelligences in the "universes next door" that
can and do intrude upon us. Sarfatti]