Function core_function 

pub fn core_function(
    name: &str,
    args: &Map<String, Value>,
) -> Option<Option<Value>>

The core value functions (core.catalog.json functions.core; set is an action, data::run_action) once the arguments are resolved: the reference every client function table mirrors (src/expr.ts CORE_FUNCTIONS). Outer None = not a core function; inner None = undefined.