  Symbol alphabets of n-particle amplitudes in N=4 super-Yang-Mills theory are
known to contain certain cluster variables of Gr(4,n) as well as certain
algebraic functions of cluster variables. In this paper we solve the C Z = 0
matrix equations associated to several cells of the totally non-negative
Grassmannian, combining methods of arXiv:2012.15812 for rational letters and
arXiv:2007.00646 for algebraic letters. We identify sets of parameterizations
of the top cell of Gr_+(5,9) for which the solutions produce all of (and only)
the cluster variable letters of the 2-loop nine-particle NMHV amplitude, and
identify plabic graphs from which all of its algebraic letters originate.
