The contract is the unit of code.

contract PaymentService {
    vow currency: String = "USD"

    subcontract Validation {
        pledge validate_amount(amount: Float) -> Result<Bool, Int> {
            if amount > 0.0 {
                return Ok(true)
            }
            return Err(1)
        }
    }

    subcontract Processing {
        // Abstract: the host binds this before sign.
        pledge charge(card: String, amount: Float) -> Result<Bool, Int>
    }

    requirements {
        fulfill: Validation && Processing
        partial: Validation || Processing
        break: !Validation && !Processing
    }
}

from geas import Runtime, Ok, Err

def charge(inst, args):
    card, amount = args
    return Ok(True) if amount > 0 else Err(41)

with Runtime("libgeasrt.so") as rt:
    rt.load("libpayment.geas.so")
    rt.bind("PaymentService.charge", charge)
    rt.freeze()

    c = rt.sign("PaymentService", vows={"currency": "EUR"})
    c.fulfill_sync("validate_amount", 25.0)
    c.fulfill_sync("charge", "4111 1111", 25.0)

    p = c.partial()
    # p.fulfilled == ["Validation", "Processing"]
    # the instance is Fulfilled, the terms held