private async Task<Document> MakePrivateAsync(Document document, FieldDeclarationSyntax fieldDeclaration, CancellationToken cancellationToken) {
SyntaxToken privateSyntaxToken = CreatePrivateSyntaxToken(fieldDeclaration);
IEnumerable<SyntaxToken> newFieldModifiers = new[] { privateSyntaxToken }.Union(
fieldDeclaration.Modifiers.Where(modifier =>
modifier.Kind() != SyntaxKind.InternalKeyword && modifier.Kind() != SyntaxKind.PublicKeyword)
.Select(modifier => modifier.WithLeadingTrivia()));
FieldDeclarationSyntax newfieldDeclaration = fieldDeclaration.Update(fieldDeclaration.AttributeLists,
new SyntaxTokenList().AddRange(newFieldModifiers),
fieldDeclaration.Declaration.WithoutTrivia(),
fieldDeclaration.SemicolonToken)
.WithLeadingTrivia(fieldDeclaration.GetLeadingTrivia())
.WithTrailingTrivia(fieldDeclaration.GetTrailingTrivia());
return await this.ReplaceNodeInDocumentAsync(document, cancellationToken, fieldDeclaration, newfieldDeclaration);
}