code: function _flattenBtcBranches(proofs) {
    let flattenedBranches = []
  
    _forEach(proofs, proof => {
      let btcAnchor = {}
      btcAnchor.hash_id_node = proof.hash_id_node
  
      if (proof.branches) {
        _forEach(proof.branches, branch => {
          // sub branches indicate other anchors
          // we want to find the sub-branch that anchors to btc
          if (branch.branches) {
            // get the raw tx from the btc_anchor_branch
            let btcBranch = branch.branches.find(
              element => element.label === 'btc_anchor_branch'
            )
            btcAnchor.raw_btc_tx = btcBranch.rawTx
            // get the btc anchor
            let anchor = btcBranch.anchors.find(anchor => anchor.type === 'btc')
            // add expected_value (i.e. the merkle root of anchored block)
            btcAnchor.expected_value = anchor.expected_value
            // add anchor_id (i.e. the anchored block height)
            btcAnchor.anchor_id = anchor.anchor_id
          }
        })
      }
  
      flattenedBranches.push(btcAnchor)
    })
  
    return flattenedBranches
  }
docs: Get raw btc transactions for each hash_id_node
  @param {Array} proofs - array of previously parsed proofs
  @return {Obect[]} - an array of objects with hash_id_node and raw btc tx