A Word is represented by a set of observations and a set of potential interpretation labels. When the set of potential labels is empty the word is considered to be unknown and the default potential set is used in its place.
The word is considered to be unknown when the set of potential labels is empty.
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.