reserve x,y,y1,y2,z,a,b for object, X,Y,Z,V1,V2 for set,
  f,g,h,h9,f1,f2 for Function,
  i for Nat,
  P for Permutation of X,
  D,D1,D2,D3 for non empty set,
  d1 for Element of D1,
  d2 for Element of D2,
  d3 for Element of D3;

theorem Th50:
  for A,B,C be set, f be Function st A <> {} & B <> {} & f in
  Funcs(A,Funcs(B,C)) holds commute f in Funcs(B,Funcs(A,C))
proof
  let A,B,C be set, f be Function;
  assume A <> {} & B <> {} & f in Funcs(A,Funcs(B,C));
  then [:A,B:] <> {} & uncurry f in Funcs([:A,B:],C) by Th11,ZFMISC_1:90;
  hence thesis by Th10;
end;
