# Introduction Through Proof of Transfer the Stacks blockchain has a view on the Bitcoin network. It is limted to block header hashes of each Bitcoin block. This includes even those blocks that do not have a corresponding Stacks block e.g. for flash blocks. The block header hash is sufficient to proof in a Clarity contract that a Bitcoin transaction was actually included in the Bitcoin block. It is just a question about providing enough information to re-build the hash. This also works for transaction with witness data. Currently, a stateless contract is deployed on mainnet that helps to make use of this feature for transactions with and without witness data, but limited to transactions with a smaller number of inputs and outputs: The next sections describe the details of the library and its application. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://clarity-bitcoin.gitbook.io/clarity-bitcoin-library/introduction.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.