Fuzzy boolean
From FLWiki
A classical boolean value can only take on two values: True and False. No other values are permitted. By contrast, a fuzzy boolean value can take on a continuous range of values including True, False, and any shade in between. This allows a proposition to be asserted with some degree of ambiguity or vagueness.
By default, Flying Logic documents are set up to treat confidence values as fuzzy booleans. This means at a minimum that you have access to a third indeterminate value (neither True nor False) when checking the logic of your diagrams. An indeterminate confidence value on an entity can either represent "no opinion" or "conflicting opinions" on the truth of its proposition.
Statements such as, "The water in the pot is hot enough," are fuzzy in nature. Note that the statement expresses certainty, and so is not talking about the probability of the water being hot enough, rather it is being vague about what hot enough-ness is.
Internally, fuzzy booleans are represented as numerical values from 0.0 ... 1.0, with 0.0 representing False and 1.0 representing True. If an operator results in a value outside this range, the number is "promoted" from a fuzzy boolean type to a floating-point type. Edges that carry fuzzy boolean values are drawn with filled arrowheads, while edges that carry floating-point values are drawn with open arrowheads.
