$$ \matrix{a & b  \cr c & d} $$
$$ \bordermatrix{ & A & B \cr C & 0 & 1 \cr D & 2 & 3} $$
$$ \pmatrix{a & b  \cr c & d} $$
$$ s(x) = \cases{+1 & whatever\cr -1 & $x<0$ \cr 0} $$
$$ \eqalign{ x  &\ll y+i + \cdots + y_n \cr
                &\leq  z } $$
$$ \eqalign{ x + y + z  &\ll  y+i + \cdots + y_n \cr
                & \leq q } $$
$$ \eqalignno{ x  &\ll  y+i + \cdots + y_n & (x)\cr
                 & \leq  z } $$
$$ \leqalignno{ x  &\ll  y+i + \cdots + y_n & (x)\cr
                 & \leq  z } $$
\bye