gistfile1.cs

private static IEnumerable<UnionVisitor<TQuerySource>.UnionExpression> GetUnionExpressions(Expression expression)
{
    Func<Expression, IEnumerable<UnionVisitor<TQuerySource>.UnionExpression>> getVisitorExpressions = x =>
                                {
                                    var unionVisitor = new UnionVisitor<TQuerySource>();
                                    unionVisitor.Visit(x);
                                    return unionVisitor.Expressions;
                                };
    return GetUnionExpressions(getVisitorExpressions(expression), getVisitorExpressions);
}

private static IEnumerable<UnionVisitor<TQuerySource>.UnionExpression> GetUnionExpressions(
    IEnumerable<UnionVisitor<TQuerySource>.UnionExpression> expressions, 
    Func<Expression, IEnumerable<UnionVisitor<TQuerySource>.UnionExpression>> getExpressions)
{
    return expressions.Any() ? expressions.Union(GetUnionExpressions(expressions.SelectMany(x => getExpressions(x.Expression)), getExpressions)) : 
                                Enumerable.Empty<UnionVisitor<TQuerySource>.UnionExpression>();
}