theorem Th48: :: MATRIX_0:48

for x1, x2, y1, y2 being object holds
( [1,1] in Indices ((x1,x2) ][ (y1,y2)) & [1,2] in Indices ((x1,x2) ][ (y1,y2)) & [2,1] in Indices ((x1,x2) ][ (y1,y2)) & [2,2] in Indices ((x1,x2) ][ (y1,y2)) )