What is the difference between an atomic pattern and a pattern with elements?

An atomic pattern holds a value and has no elements. It is a leaf — there is nothing beneath it.

A pattern with elements holds a value and has one or more sub-patterns as elements. The elements are ordered and each is itself a Pattern<V>.

In Gram notation:

• (alice:Person) is an atomic pattern. The node has a Subject value and no elements.
• (alice:Person)-[:KNOWS]->(bob:Person) expands to a pattern with two elements: the alice pattern and the bob pattern, decorated by the relationship subject.

The is_atomic() predicate tests whether a pattern has no elements:

let p = Pattern::point(subject);
assert!(p.is_atomic()); // true — no elements

let q = Pattern::pattern(subject, vec![p]);
assert!(!q.is_atomic()); // false — has one element

Atomic patterns are the base case for all recursive operations. map, fold, and para reach the atomic patterns and return. A pattern entirely composed of atomic patterns in its elements list has depth 1.