Function document_count_of_element 

pub fn document_count_of_element(index: &Index, element: &Element) -> u64

The number of documents an element a count read of index reaches stands for: its count, or, on a summableOffCountIndex index, its sum. Such an index keeps one counter per group, which counts one in its count trees and adds its group’s documents to their sums, and its registration rules make each counter equal its source group’s entries, so its sums are the document counts.

Unversioned, so every protocol version reaches it: it departs from the plain count only on a summableOffCountIndex index, which only meta-schema v3 (protocol version 14) admits.