The spec defines three string built-in functions that are not yet implemented:

• string_length(@String.0) — returns Nat
• string_concat(@String.0, @String.1) — returns String
• string_slice(@String.0, @Nat.0, @Nat.1) — returns String

Spec reference: Chapter 4 (Expressions), Section 4.7 — marked as "Status: Not yet implemented. Depends on dynamic string construction (#52)."

Depends on #52 (dynamic string construction) for string_concat and string_slice which produce new String values at runtime. string_length could potentially be implemented independently.