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A *domain* is *continuous* if for every action `a` there exist
two actions `a_1` and `a_2` such that the result of the composite action
`a_1 a_2` is the same as the result of `a`.

An action `a` is *elementary* if `a \not= a_1 a_2` for any two
actions `a_1` and `a_2`.

A domain is *discrete* if every action from it can be represented
by a finite sequence of elementary actions.

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