Obsecure problem 1

[pascal111 via Digitalmars-d-learn]

I've typed a code to enjoy with my library "dcollect", and found 
non-understandable error:

module main;
import std.stdio;
import dcollect;
import std.string;
import std.conv;


int main(string[] args)
{

        string sentence_x,
        sent_result, token2;
        string[] sentence_tokens;

        writeln(strstring(5,"*")," No tail \"s\" 
",strstring(5,"*"));
        writeln;

        write("Compose a line: ");
        sentence_x=strip(readln());

        sentence_tokens=sentence_x.d_strtok(" ,;.:!?\"'");

        foreach(token; sentence_tokens){
            token2=token.strrtrim;
            if(token2[token2.length-1]=='s')
                token2=strdel(token2, token2.length-1,1);
                sent_result~=token2~" ";}

        sent_result.writeln;

        return 0;
    }


dcollect library:
https://github.com/pascal111-fra/D/blob/main/dcollect.d

I changed some data types of some functions parameters to "ulong":

string strdel(const string ch, ulong x, ulong l )
{

        string l_ch=ch.strleft(x);
        string r_ch=ch.strright(ch.length-(x+l));

        return (l_ch~=r_ch);

}

string strleft(const string ch, ulong n)
{

        string ch_sub;

        ch_sub=ch[0..n];

        return ch_sub;

}

/************************************/

string strreverse(const string ch)
{

        string ch_rev;

        for(ulong i=ch.length-1; i>=0; i--)
            ch_rev~=ch[i];


        return ch_rev;


}

/*********************************************/

string strright(const string ch, ulong n)
{

        string ch_sub1,
        ch_sub2;

        ch_sub1=strreverse(ch);

        ch_sub2=strleft(ch_sub1, n);

        ch_sub1=strreverse(ch_sub2);

        return ch_sub1;

}

====================

Running screen says:

https://i.postimg.cc/G3YyCmbF/Screenshot-from-2022-07-30-23-23-59.png