token infix:sym<?? !!> ( --> Conditional) {
    '??'
    <EXPR(%conditional)>
    [ '!!' ||
        [
        || <?before '='> <panic: Assignment not allowed within ??!!>
        || <?before '::'> <panic: Please use !! rather than ::>
        || <?before <infix>>    # Note: a tight infix would have parsed right
            <panic: Precedence too loose within ??!!; use ??()!! instead >
        || <panic: Found ?? but no !!; possible precedence problem>
        ]
    ]
    {*}                                                         #= ?? !!
}

##</code><code>##

token infix:sym<?? !!> {
    '??'
    <EXPR(%conditional)>
    '!!'
}

##</code><code>##

$a == 1 ?? $b = 2 !! $b = 3;

##</code><code>##

EXPR INFIX EXPR LEFT_DELIM STRING RIGHT_DELIM

##</code><code>##

for 1..10 { ... }