Not the kind of sequencer I was first expecting[0], but I'll take it.
But, anyway .. Here's another amazing open source sequencer, hard and soft, worthy of the attention of the sequencer-loving subdivision of the HN music-making aristocracy:
https://github.com/FundamentalFrequency/
Yet to build one myself, but having immense fun with the hardware emulator that is included in the project .. definitely worth the build.