pub const UD: Term;Expand description
An undefined term that can be used as a value returned by invalid/inapplicable operations, e.g.
obtaining an element of an empty list. Since this implementation uses De Bruijn indices greater
than zero, Var(0) will not occur naturally.
fmt::Display shows it as undefined; fmt::Debug shows it as [0], which is the form
parse reads back, so a term holding it still round-trips.