x_{1} (li) is the cardinality of set x_{2} (set)

Exactly equivalent to nilzilcmi except in morphology. But cardinality is a pretty fundamental thing in mathematics (in some sense, even more fundamental than natural numbers), so it arguably deserves a gismu; this will also shorten many lujvo which should use a word for cardinality. The second terbri should technically be filled by a set, but other structures could probably be acceptable; these might include groups and other algebraic structures, multisets, lists, classes, vel sim. ci'i alone (without being followed by another (integer-representing) digit) indicates an infinite cardinality but not which one; beware the assumption of the Axiom of Choice or Generalized Continuum Hypothesis.