public async Task <string> SaveAsync(string fileName, IReadOnlyCollection <LineToSave> linesToSave)
{
var snapshot = workspaceManager.GetHistoricalSnapshot();
var allNodes = await GetNodes(snapshot);
var usings = GetUsingDirectives(snapshot, allNodes);
// our placeholder program - we use a placeholder because it's unlikely all the
// expressions a user can put into the REPL will be embeddable into a main method.
// So we use a string placeholder replacement, for a "best effort" approach.
const string PlaceholderText = "var PLACEHOLDER = 42;";
var compilation = BuildPlaceholderFile(usings, PlaceholderText);
var document = CreateDocument(compilation);
// the contents to swap into the placeholder
var statements = CreateStringFromStatements(allNodes.Except(usings));
// do the swap
var newText = compilation.ToFullString().Replace(PlaceholderText, statements);
document = document.WithText(SourceText.From(newText));
// make it pretty
var formatted = await Formatter.FormatAsync(document);
var text = await formatted.GetTextAsync();
// write to file
await io.WriteAllLinesAsync(fileName, new[] { text.ToString() }, Encoding.UTF8);
return("Session has been saved as a C# file. Please note that not everything in a REPL "
+ "translates nicely to a C# file, so you may need to fix up the file.");
}