theorem Th48:
  [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)
proof
A1: 2 in Seg 2 by FINSEQ_1:2,TARSKI:def 2;
  Indices (x1,x2)][(y1,y2)=[:Seg 2,Seg 2:] & 1 in Seg 2 by Th47,FINSEQ_1:2
,TARSKI:def 2;
  hence thesis by A1,ZFMISC_1:87;
end;
