snippet.swift

let first = 0
let last = dataSource.lines.lines.count
        
let missing = dataSource.lines.computeMissing(first, last)
for (f, l) in missing {
    Swift.print("requesting missing: \(f)..\(l)")
    // output: requesting missing: 0..1
    _ = dataSource.document.sendRpc("request_lines", params: [f, l])
}

let line = getLine(0) // line == nil, but why?