  Agreements and contracts are everywhere, but they are built on layers and
layers of legal and social institutions. Software is slowly entering into this
stack. In this article, we introduce agreement paths, a general model for
understanding and decomposing digital agreement systems, and Agreement Engine,
an open-source software service for building net-native agreement systems. We
demonstrate Agreement Engine by building two example agreement systems: Scarce
Knowledge, an app for crowdfunding essays, and Twitter Social Capital, a bot
that allows users to form and enforce Twitter agreements.
