Machinist.from

You're seeing just the macro from, go back to Machinist module for more information.
Link to this macro

from(state, list)

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Defines a state transition with the given state, and the list of options [to: new_state, event: event]

from 1, to: 2, event: "next"

It's also possible to define a from any state transition to another specific one, by just passing an underscore variable in place of a real state value

from _state, to: :expired, event: "enrollment_expired"