extension<T>(ICollection<T>)
{
    public static bool operator >(T elem, ICollection<T> collection) => collection.Contains(elem);
    public static bool operator <(T elem, ICollection<T> collection) => !collection.Contains(elem);
}

var list = new List<int> {1, 2, 3};
if (1 | list)
{
    // it in there
}

public static class CollectionExtensions
{
    extension<T>(ICollection<T>)
    {
        public static bool operator |(T elem, ICollection<T> collection) => collection.Contains(elem);
    }
}