Define how nodes select a best chain. fixes #97

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

protocol/protocol.tex

@@ -1136,6 +1136,12 @@ Each \block in a \blockchain has a \blockHeight. The \blockHeight of the
 \genesisBlock is 0, and the \blockHeight of each subsequent \block in the
 \blockchain increments by 1.
 
+In order to choose the ``best'' valid \blockchain in its view of the overall
+\block tree, a node sums the work, as defined in \crossref{workdef}, of
+all \blocks in each chain, and considers the chain with greatest total work
+to be best. To break ties between leaf \blocks, a node will prefer the
+\block that it received first.
+
 The consensus protocol is designed to ensure that for any given \blockHeight,
 the vast majority of nodes should eventually agree on their ``best''
 \blockchain up to that height.
@@ -3441,6 +3447,18 @@ fully defined in the Bitcoin documentation \cite{Bitcoin-nBits}, and so we defin
         \end{cases}$
 \end{formulae}
 
+\introlist
+\nsubsubsection{Definition of Work} \label{workdef}
+
+As explained in \crossref{blockchain}, a node chooses the ``best'' \blockchain
+visible to it by finding the chain of valid \blocks with the greatest total work.
+
+Let $\ToTarget$ be as defined in \crossref{nbits}.
+
+The work of a \block with value $\nBits$ for the $\nBitsField$ field
+in its \blockHeader is defined as $\floor{\hfrac{2^{256}}{\ToTarget(\nBits) + 1}}$.
+
+
 \introlist
 \nsubsection{Calculation of Block Subsidy and Founders' Reward} \label{subsidies}
 
@@ -4073,6 +4091,7 @@ The errors in the proof of Ledger Indistinguishability mentioned in
     \item Make the description of \blockchains more consistent with
           upstream \Bitcoin documentation (referring to ``best`` chains
           rather than using the concept of a \term{block chain view}).
+    \item Define how nodes select a best chain.
 \end{itemize}
 
 \introlist