[Node to be completed]
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.