text: const withPrevious = source => (start, sink) => {
    if (start !== 0) return;
    let hasPrevious = false;
    let previous;
    source(0, (t, d) => {
      let data = d;
      if (t === 1){
        data = hasPrevious ? [d, previous] : [d, undefined, true];
        previous = d;
        hasPrevious = true;
      }
      sink(t, data);
    });
  };
  
  module.exports = withPrevious;
  