code: Node getBranchNode(int lineNumber, int branchNumber) {
      Preconditions.checkArgument(
          lineNumber > 0, "Expected non-zero positive integer as line number: %s", lineNumber);
      Preconditions.checkArgument(
          branchNumber > 0, "Expected non-zero positive integer as branch number: %s", branchNumber);
  
      return branchNodes.get(BranchIndexPair.of(lineNumber - 1, branchNumber - 1));
    }
docs: Get the block node to be instrumented for branch coverage.
  @param lineNumber 1-based line number
  @param branchNumber 1-based branch number
  @return the node of the conditional block.