Text rearranging, C#

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;
using System.Diagnostics;

namespace Rextester
{
    public class Program
    {
        static string Simplify(string[] pieces)
        {
            var stopWords = new HashSet<string>()
            {
                "PROVIDE",
                "NEED",
            };
            int newLength = pieces.Length;
            if (pieces.Last() == "AND") --newLength;
            var keepPieces = pieces.Take(newLength).TakeWhile(p => !stopWords.Contains(p));
            return String.Join(" ", keepPieces);
        }

static List<string> Subchoices(string[] pieces)
        {
            var results = new List<string>();

if (pieces.Length == 0)
            {
                return results;
            }
            
            //-- Check for base cases, no more ORS
            int nextOR = Array.IndexOf(pieces, "OR");
            if (nextOR == -1)
            {
                results.Add(Simplify(pieces));
            } else
            {
                var beforePieces = pieces.Take(nextOR).ToArray();
                var afterPieces = pieces.Skip(nextOR + 1).ToArray();
                var afterPaths = Subchoices(afterPieces);
                //var beforeString = Simplify(beforePieces);
                results.Add(Simplify(beforePieces));
                results.AddRange(afterPaths.Select(ap => ap));
            }
            return results;
        }

static List<string> Transform(string[] pieces)
        {
            string[] initialRelations = new string[] { "REQUIRE", "PROVIDE", "NEED" };
            Debug.Assert(pieces.Length > 2, "String not valid relation");
            Debug.Assert(initialRelations.Contains(pieces[1]), "Invalid initial relation");
            string firstEntity = pieces[0];
            string firstRelation = pieces[1];
            var result = Subchoices(pieces.Skip(2).ToArray()).Select(choice =>
            {
                return firstEntity + " " + firstRelation + " " + String.Join(" ", choice);
            }).ToList();
            return result;
        }
        
        public static void Main(string[] args)
        {
            String A = "Passager NEED 1 Passagersæde OR  1 Taxasæde NEED 500 Kassebeholdning OR ";
            String B = "Train REQUIRE 1 Trinbræt OR  Station AND  Track AND  OR "; ;
            String C = "Train PROVIDE 1 Passagervogn PROVIDE 250 Passagersæde OR  1 Cykelvogn PROVIDE 200 Passagersæde Cykelvogn PROVIDE 30 Cykelplads OR ";
            foreach (var input in new string[] { A, B, C})
            {
                var result = Transform(input.Split(new char[] { ' ' }, StringSplitOptions.RemoveEmptyEntries));
                Console.WriteLine("Input: {0}", input);
                Console.WriteLine("\t\t yields");
                foreach (var x in result)
                {
                    Console.WriteLine("\t" + x);
                }
                Console.WriteLine();
            }
        }
    }
}

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Compilation time: 0,16 sec, absolute running time: 0,11 sec, cpu time: 0,09 sec, average memory usage: 15 Mb, average nr of threads: 3