  We describe a general algorithm which builds on several pieces of data
available in the literature to construct explicit analytic formulas for
two-loop MHV amplitudes in N=4 super-Yang-Mills theory. The non-classical part
of an amplitude is built from $A_3$ cluster polylogarithm functions; classical
polylogarithms with (negative) cluster X-coordinate arguments are added to
complete the symbol of the amplitude; beyond-the-symbol terms proportional to
$\pi^2$ are determined by comparison with the differential of the amplitude;
and the overall additive constant is fixed by the collinear limit. We present
an explicit formula for the seven-point amplitude $R_7^{(2)}$ as a sample
application.
