Three-bit example. Consider again the theory described by the state space
Ω of three bits, and all transformations on those bits. (
a) All operations
that change the third bit (of which id and
notC are labelled). (
b) Equivalence classes
built according to definition . These correspond to the view of an agent who can only distinguish the first two bits. The equivalence relation
, which coarse-grains over the functions applied to the third bit, gives us the largest effective state space relative to which functions in
are secret. (
c) More coarse-grained equivalence classes [
x]
A (vertical, yellow) and [
x]
B (horizontal, blue), corresponding to an agent
A who can only distinguish the first bit and an agent
B who only sees the second bit, respectively. Operations in
are still secret relative to these two agents. In addition, operations on the first bit are secret towards
B and vice versa. These smaller effective state spaces correspond to equivalence relations on the effective state space
(as in the nested agents of proposition ). The two-bit space
is a
common state space of
A and
B, including states that could be distinguished if the two agents could work together, with
. (Online version in colour.)