External data representation.
A word consists of a set of observations and a set of potential labels.
A word constructor which checks non-emptiness of the potential set of labels.
A probability distribution defined over elements of type a. All elements not included in the map have probability equal to 0.
A WordL is a labeled word, i.e. a word with probability distribution defined over labels. We assume that every label from the distribution domain is a member of the set of potential labels corresponding to the word. TODO: Ensure the assumption using the smart constructor.