perl6 grammar

gistfile1.txt

I'm trying to use a perl6 grammar inside another one. You will find my testcase below. Unfortunately, when running this code,I get the error message :
Cannot assign to a readonly variable or a value
  in method TOP at ./sub021.pl6 line 41
  in regex TOP at ./sub021.pl6 line 30
  in block  at /opt/rakudo-star-2016.10/share/perl6/site/sources/88A197C57F6C4623C1071F68DFC42E86CA5F4B4D (Grammar::Tracer) line 21
  in block <unit> at ./sub021.pl6 line 53
I dont understand why my variable is read only... 

#!/opt/rakudo-star-2016.10/bin/perl6

use v6;
use Grammar::Tracer;

 
grammar NVL {

    rule  TOP    { [<main>]* <textfin> }
      rule  main   { <text>  <nvl> }
        regex text     {:s .+? }
        rule  nvl    {:i   'nvl' '(' <expr1> ',' <expr2> ')'}
          regex expr1     { .+? }
          regex expr2     { .+? }
      regex textfin     { .+ }
}


class Nvl {

  method TOP ($/) { my   $result ~= make $_.made for $<main>;
                    make $result ~= $<textfin>; }
  method main ($/) { make $<text> ~ $<nvl>.made ; }
  method nvl ($/) { make "coalesce("~$<expr1>~','~$<expr2> ~ ")" ;}
}

 
grammar TEST {

rule  TOP     {  <sql> }
  rule  sql     { \s* <sqlstart> <sqltext> <sqlend> \s* }
    token sqlstart  {'EXEC SQL'}
    token sqlend  {'END-EXEC'}
    regex sqltext     { .+? <before 'END-EXEC'> }
}

class Test {

  method TOP ($/) { my    $sql=$<sql><sqltext>.made;
                    my    $act = Nvl.new;
                    my      $result= NVL.parse($sql, :act).made ;
                  }
  method sql ($/) {  $/.make: (~$/)  }
  method sqltext ($/) {  $/.make: (~$/)  }
}

 
 


my $cbl = "EXEC SQL select nvl(toto,titi) from dual END-EXEC";

say TEST.parse($cbl, actions => Test).made;



coomplete execution results is :
 
TOP
|  sql
|  |  sqlstart
|  |  * MATCH "EXEC SQL"
|  |  sqltext
|  |  * MATCH "select nvl(toto,titi) from dual "
|  |  sqlend
|  |  * MATCH "END-EXEC"
|  * MATCH "EXEC SQL select nvl(toto,titi) from dual END-EXEC"
|  TOP
|  |  main
|  |  |  text
|  |  |  * MATCH "select "
|  |  |  nvl
|  |  |  |  expr1
|  |  |  |  * MATCH "t"
|  |  |  |  expr2
|  |  |  |  * MATCH "t"
|  |  |  * MATCH "nvl(toto,titi)"
|  |  * MATCH "select nvl(toto,titi) "
|  |  main
|  |  |  text
|  |  |  * MATCH "from "
|  |  |  nvl
|  |  |  * FAIL
|  |  |  nvl
|  |  |  * FAIL
|  |  |  nvl
|  |  |  * FAIL
|  |  |  nvl
|  |  |  * FAIL
|  |  * FAIL
|  |  textfin
|  |  * MATCH "from dual "
|  * MATCH "select nvl(toto,titi) from dual "
Cannot assign to a readonly variable or a value
  in method TOP at ./sub021.pl6 line 41
  in regex TOP at ./sub021.pl6 line 30
  in block  at /opt/rakudo-star-2016.10/share/perl6/site/sources/88A197C57F6C4623C1071F68DFC42E86CA5F4B4D (Grammar::Tracer) line 21
  in block <unit> at ./sub021.pl6 line 53

MAny thanks for your help.