for D being non empty set
for i0, j0, n0, m0 being non zero Nat
for A being Matrix of D st i0 < j0 & n0 < m0 holds
Segm (A,{i0,j0},{n0,m0}) = ((A * (i0,n0)),(A * (i0,m0))) ][ ((A * (j0,n0)),(A * (j0,m0)))