reserve x,y,z for object,
  i,j,n,m for Nat,
  D for non empty set,
  s,t for FinSequence,
  a,a1,a2,b1,b2,d for Element of D,
  p, p1,p2,q,r for FinSequence of D;
reserve M,M1,M2 for Matrix of D;
reserve f for FinSequence of D;
reserve i,j,i1,j1 for Nat;
reserve k for Nat, G for Matrix of D;
reserve x,y,x1,x2,y1,y2 for object,
  i,j,k,l,n,m for Nat,
  D for non empty set,
  s,s2 for FinSequence,
  a,b,c,d for Element of D,
  q,r for FinSequence of D,
  a9,b9 for Element of D;

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;
