YAMLish failure `foo: no good` doesn't parse

gistfile1.txt

#! /usr/bin/env perl6

grammar Grammar {
    rule TOP { <element> }
    token plain { \N* }
    token inline { yes | no }
    token element {
        <value=inline> || <value=plain>
    }
}

use Data::Dump;
for 'no', 'no bar', 'xno bar' {
    note "\n$_";
    Grammar.parse($_);
    dd $/;
    dd $/<element>.Str;
}