Returns a reverse index to map match keys back to their original keys
function buildReverseIndex(obj) {
    let warnCollisions = [];

    for (let k in obj) {
        checkAmbiguous(k);

        if (obj[k].match) {
            for (let i = obj[k].match.length - 1; i >= 0; i--) {
                let match = obj[k].match[i];
                checkAmbiguous(match);

                if (rIndex[match]) {
                    warnCollisions.push([rIndex[match], match]);
                    warnCollisions.push([k, match]);
                }
                rIndex[match] = k;
            }
        }
    }

    if (warnCollisions.length) {
        console.warn(colors.yellow('\nWarning - match name collisions'));
        console.warn('To resolve these, make sure multiple entries do not contain the same "match" property.');
        warnCollisions.forEach(w => console.warn(
            colors.yellow('  "' + w[0] + '"') + ' -> match? -> ' + colors.yellow('"' + w[1] + '"')
        ));
    }
}