reserve x for set,
  D for non empty set,
  k,n,m,i,j,l for Nat,
  K for Field;
reserve n,i,j for Nat;
reserve n for Nat;

theorem
  Inv(1_Rmatrix(n))=1_Rmatrix(n)
proof
  (1_Rmatrix(n))*(1_Rmatrix n)=1_Rmatrix(n) by Th70;
  hence thesis by Def6;
end;
