flatmap api?

flatten.js

const o = emitterify()
    , stream1 = emitterify()
    , stream2 = emitterify()

// without
o.on('primary')
 .map(d => console.log(d)) // stream1, stream2

// with
o.on('primary')
 .map(flatten('secondary'))
 .map(d => console.log(d)) // 1a, 1a, 2a, 2b


o.emit('primary', stream1)
 .emit('primary', stream2)

stream1.emit('secondary', '1a')
stream1.emit('secondary', '1b')
stream2.emit('secondary', '2a')
stream2.emit('secondary', '2b')