grammar that hangs, but works when tracer is used

.p6

#!/usr/bin/env perl6

use v6;

# XXX this makes it work
#use Grammar::Tracer;

grammar JavaPackagesAndImports {
    rule TOP { <compilationUnit> }
# switching this for the next rule also makes it work
#    rule compilationUnit { <packageDeclaration>? <STUFF>? }
    rule compilationUnit { <packageDeclaration>? <importDeclaration>* <STUFF>? }
    rule packageDeclaration { 'package' <package> ';' }
    rule package { <Identifier> [ '.' <Identifier> ]* }
    rule importDeclaration { <singleTypeImportDeclaration> | <typeImportOnDemandDeclaration> | <singleStaticImportDeclaration> | <staticImportOnDemandDeclaration> }
    rule singleTypeImportDeclaration { 'import' <typeName> ';' }
    rule typeImportOnDemandDeclaration { 'import' <packageOrTypeName> '.' '*' ';' }
    rule singleStaticImportDeclaration { 'import' 'static' <typeName> '.' <Identifier> ';' }
    rule staticImportOnDemandDeclaration { 'import' 'static' <typeName> '.' '*' ';' }
    rule Identifier { <[a..zA..Z\$_]><[a..zA..Z0..9\$_]>* }
    rule typeName { <Identifier> | <packageOrTypeName> '.' <Identifier> }
    rule packageOrTypeName { <Identifier> | <packageOrTypeName> '.' <Identifier> }
    token STUFF { .* }
}

my $code = q:heredoc/EOL/;
package com.cisco.spvss.p2o.datamodel.serialization;

import com.cisco.spvss.p2o.datamodel.common.BaseDataModel;

/** common interface for per-datamodel serializers */
public interface BaseModelSerializer {
    /** take a data model item and return a string representation. returns null if the item type
     * is not supported by this serializer, throws in case of technical problems */
    String serialize(BaseDataModel item) throws Exception;
    /** take a string representation and return a data model item. returns null if the item type
     * is not supported by this serializer, throws in case of technical problems */
    BaseDataModel deserialize(String input) throws Exception;
}
EOL
JavaPackagesAndImports.parse($code);
say "done";