_transform(chunk, _encoding, callback) {
        const prefix = this.prefix
        const nPrefix = `\n${prefix}`
        const state = this.state
        const firstPrefix =
            state.lastIsLinebreak ? prefix :
            (state.lastPrefix !== prefix) ? "\n" :
            /* otherwise */ ""
        const prefixed = `${firstPrefix}${chunk}`.replace(ALL_BR, nPrefix)
        const index = prefixed.indexOf(prefix, Math.max(0, prefixed.length - prefix.length))

        state.lastPrefix = prefix
        state.lastIsLinebreak = (index !== -1)

        callback(null, (index !== -1) ? prefixed.slice(0, index) : prefixed)
    }
Transforms the output chunk.

@param {string|Buffer} chunk - A chunk to be transformed.
@param {string} _encoding - The encoding of the chunk.
@param {function} callback - A callback function that is called when done.
@returns {void}