Layer One dot Five (L15) is an open source, decentralized protocol over Bitcoin that utilizes a construct similar to the Lightning channel as a limited trustless lockbox for a sidechain. It can also be described as a sidechain-enabled iteration of the Lightning Network, where the Lightning commitment data is recorded in the sidechain. Thus, L15 provides the ability to create Lightning-style transactions where the outcomes depend on the sidechain consensus rules.
Compared to the original two-way pegged sidechains, L15's Lightning-style approach mitigates the trustless lockbox issue, but it also imposes some limitations on the types of smart contracts it can support. Just like a Lightning channel, a particular L15 contract, for example, can interact only with a predefined amount of Bitcoin. Another useful feature is that L15 protocol allows payer nodes to go offline while their payment instructions are being honored, thus enabling trustless subscription payments.
A more complex example is to create a stablecoin minted by the L15 sidechain as a loan and collateralized by Bitcoin. This can help resolve the emerging contradiction that the Lightning network has to face: while Bitcoin is strengthening its position as a value-preserving "digital gold" that nobody wants to spend, the Lightning Network is there to facilitate fast spontaneous transactions. Making L15 is compatible with the Lightning network gives exiting opportunity: Bitcoin from the Lightning network channels can be sent to L15 contracts as collateral. The stablecoin generated as a result can be used for the Lightning payments, thus both preserving the "store of value" paradigm for Bitcoin and supporting the Lightning transactional capabilities at the same time.