using System;
using System.Collections.Generic;

public sealed class SymbolTable
{
    private readonly HashSet<string> _allowed;

    public SymbolTable(IEnumerable<string> allowed)
    {
        if (allowed is null) throw new ArgumentNullException(nameof(allowed));
        _allowed = new HashSet<string>(StringComparer.Ordinal);
        foreach (var t in allowed)
        {
            if (t is null) continue;
            _allowed.Add(string.Intern(t)); // pre-canonicalize
        }
    }

    public string Canonicalize(string text)
    {
        if (text is null) throw new ArgumentNullException(nameof(text));
        // Only intern when the vocabulary is known and bounded
        return _allowed.Contains(text) ? string.Intern(text) : text;
    }

    public bool AreSameSymbol(string a, string b) =>
        object.ReferenceEquals(Canonicalize(a), Canonicalize(b));
}

public static class Demo
{
    public static void Main()
    {
        var syms = new SymbolTable(new[] { "IF", "ELSE", "LET", "RETURN" });

        var x1 = syms.Canonicalize(new string("IF"));
        var x2 = syms.Canonicalize("IF");
        var y1 = syms.Canonicalize("WHATEVER"); // not interned (not allowed)

        Console.WriteLine(object.ReferenceEquals(x1, x2)); // True
        Console.WriteLine(syms.AreSameSymbol("IF", "IF"));  // True
        Console.WriteLine(syms.AreSameSymbol("IF", "ELSE"));// False
        Console.WriteLine(object.ReferenceEquals(y1, "WHATEVER")); // likely False (distinct), and not interned
    }
}