Explicit conversion of a scalar degree into a truth value: truth(x).
Also the printed form of a graded core._Truth (truth(0.25)), which is what makes a fuzzy degree round-trip through toString and through a :save/:load — without it a degree would print as a bare number and re-parse as a plain _Number. The Kleene midpoint keeps its own literal, unknown.
This is also how a custom membership function is written: the argument is an arbitrary scalar expression, so truth(1 / (1 + (x - 5)^2)) is a perfectly good curve and composes with the hedges, the connectives, and defuzz exactly like the built-in ones.
An argument outside [0, 1] is not a degree, so the node stays symbolic rather than clamping — the same "domain errors stay symbolic" rule the rest of the library follows, and a useful signal that a custom curve has left the unit interval.
