function merge(...sources) {
  return (start, sink) => {
    if (start !== 0) return;
    const n = sources.length;
    const sourceTalkbacks = new Array(n);
    let startCount = 0;
    let endCount = 0;
    let ended = false;
    const talkback = (t, d) => {
      if (t === 2) ended = true;
      for (let i = 0; i < n; i++) sourceTalkbacks[i] && sourceTalkbacks[i](t, d);
    };
    for (let i = 0; i < n; i++) {
      if (ended) return;
      sources[i](0, (t, d) => {
        if (t === 0) {
          sourceTalkbacks[i] = d;
          if (++startCount === 1) sink(0, talkback);
        } else if (t === 2 && d) {
          ended = true;
          for (let j = 0; j < n; j++) {
            if (j !== i) sourceTalkbacks[j] && sourceTalkbacks[j](2);
          }
          sink(2, d);
        } else if (t === 2) {
          sourceTalkbacks[i] = void 0;
          if (++endCount === n) sink(2);
        } else sink(t, d);
      });
    }
  };
}
callbag-merge
-------------

Callbag factory that merges data from multiple callbag sources. Works well
with listenable sources, and while it may work for some pullable sources,
it is only designed for listenable sources.

`npm install callbag-merge`

Example:

    const interval = require('callbag-interval');
    const forEach = require('callbag-for-each');
    const merge = require('callbag-merge');

    const source = merge(interval(100), interval(350));

    forEach(x => console.log(x))(source); // 0
                                          // 1
                                          // 2
                                          // 0
                                          // 3
                                          // 4
                                          // 5
                                          // ...