theorem Th83:
  for L, E for t being Element of L st E.t = 2
      for F, G being Polish-WFF of L, E
    holds Polish-WFF-head(Polish-binOp(L, E, t).(F, G)) = t
        & Polish-WFF-args(Polish-binOp(L, E, t).(F, G)) = <*F, G*>
proof
  let L, E;
  let t be Element of L;
  assume A1: E.t = 2;
  let F, G be Polish-WFF of L, E;
  set W = Polish-WFF-set(L, E);
  set H = Polish-binOp(L, E, t);
  set K = Polish-operation(L, E, t);
  set v = H.(F, G);
  F in W^^1 & G in W^^1;
  then F^G in W^^(1+1) by Th11;
  then reconsider u = F^G as Element of W^^(E.t) by A1;
  v = Polish-operation(L, E, t).u by A1, Def28;
  hence thesis by A1, Th64;
end;
