reserve x,y,y1,y2 for set;
reserve G for Group;
reserve a,b,c,d,g,h for Element of G;
reserve A,B,C,D for Subset of G;
reserve H,H1,H2,H3 for Subgroup of G;
reserve n for Nat;
reserve i for Integer;
reserve L for Subset of Subgroups G;
reserve N2 for normal Subgroup of G;

theorem
  for N1,N2 being strict normal Subgroup of G holds N1 /\ N2 is normal
proof
  let N1,N2 be strict normal Subgroup of G;
  let a;
  thus (N1 /\ N2) |^ a = (N1 |^ a) /\ (N2 |^ a) by Th63
    .= N1 /\ (N2 |^ a) by Def13
    .= the multMagma of (N1 /\ N2) by Def13;
end;
