reserve L,L1,L2 for Lattice,
  F1,F2 for Filter of L,
  p,q,r,s for Element of L,
  p1,q1,r1,s1 for Element of L1,
  p2,q2,r2,s2 for Element of L2,
  X,x,x1,x2,y,y1,y2 for set,
  D,D1,D2 for non empty set,
  R for Relation,
  RD for Equivalence_Relation of D,
  a,b,d for Element of D,
  a1,b1,c1 for Element of D1,
  a2,b2,c2 for Element of D2,
  B for B_Lattice,
  FB for Filter of B,
  I for I_Lattice,
  FI for Filter of I ,
  i,i1,i2,j,j1,j2,k for Element of I,
  f1,g1 for BinOp of D1,
  f2,g2 for BinOp of D2;
reserve F,G for BinOp of D,RD;

theorem Th15:
  (i/\/FI) "\/" (j/\/FI) = (i"\/"j)/\/FI & (i/\/FI) "/\" (j/\/FI)
  = (i"/\"j)/\/FI
proof
  set R = equivalence_wrt FI;
A1: j/\/FI = Class(R,j) by Def6;
  reconsider jj = join(I), mm = meet(I) as BinOp of R by Th13,Th14;
A2: i/\/FI = Class(R,i) by Def6;
A3: I /\/ FI = LattStr (#Class R, jj/\/R, mm/\/R#) by Def5;
  (i"\/"j)/\/FI = Class(R,i"\/"j) by Def6;
  hence (i/\/FI) "\/" (j/\/FI) = (i"\/"j)/\/FI by A2,A1,A3,Th3;
  (i"/\"j)/\/FI = Class(R,i"/\"j) by Def6;
  hence thesis by A2,A1,A3,Th3;
end;
