An example of custom syntax for making sets of numbers.

CustomSyntaxForNumberSets.cs

using System;
using System.Collections;
using System.Collections.Generic;
using static SetSyntax;
class Program
{
	static void Main()
	{
		var numbers = Set * 0..5 - 1..3 + 9..20 - 12..15 + 7 - 19;
		foreach (int value in numbers)
		{
			Console.WriteLine(value);
		}
	}
}
public struct SetSyntax : IEnumerable<int>
{
	public static SetSyntax Set => default;
	internal HashSet<int> Numbers;
	internal SetSyntax(HashSet<int> set) => Numbers = set;
	public static SetSyntax operator +(SetSyntax a, int b) => new SetSyntax(a.Numbers.AddTo(b));
	public static SetSyntax operator +(SetSyntax a, Range b) => new SetSyntax(a.Numbers.AddTo(b.Enumerate()));
	public static SetSyntax operator -(SetSyntax a, int b) => new SetSyntax(a.Numbers.RemoveFrom(b));
	public static SetSyntax operator -(SetSyntax a, Range b) => new SetSyntax(a.Numbers.RemoveFrom(b.Enumerate()));
	public static SetSyntax operator *(SetSyntax a, int b) => new SetSyntax(new HashSet<int>(new[] { b }));
	public static SetSyntax operator *(SetSyntax a, Range b) => new SetSyntax(new HashSet<int>(b.Enumerate()));
	public IEnumerator<int> GetEnumerator() => Numbers.GetEnumerator();
	IEnumerator IEnumerable.GetEnumerator() => Numbers.GetEnumerator();
}
public static class Extensions
{
	internal static HashSet<T> AddTo<T>(this HashSet<T> set, T value)
	{
		set.Add(value);
		return set;
	}
	internal static HashSet<T> RemoveFrom<T>(this HashSet<T> set, T value)
	{
		set.Remove(value);
		return set;
	}
	internal static HashSet<T> AddTo<T>(this HashSet<T> set, IEnumerable<T> enumerable)
	{
		enumerable.ForEach(i => set.Add(i));
		return set;
	}
	internal static HashSet<T> RemoveFrom<T>(this HashSet<T> set, IEnumerable<T> enumerable)
	{
		enumerable.ForEach(i => set.Remove(i));
		return set;
	}
	public static void ForEach<T>(this IEnumerable<T> enumerable, Action<T> action)
	{
		foreach (T v in enumerable)
			action(v);
	}
	public static IEnumerable<int> Enumerate(this Range range)
	{
		int length = range.End.Value - range.Start.Value;
		for (int i = 0; i < length; i++)
			yield return i + range.Start.Value;
	}
}