\ifx\undefined\eplain \input eplain \fi

This tests matrix inside eqalignno with leftdisplays.

% $$\eqalignno{
%  \matrix{a \cr b\cr c} & = 1 & 1 \cr
% }$$

\loggingall

\leftdisplays
$$\eqalignno{
  \matrix{a \cr b\cr c} & = 1 & 1 \cr
}$$

\bye