reserve S for non void non empty ManySortedSign,
  V for non-empty ManySortedSet of the carrier of S;
reserve A for MSAlgebra over S,
  t for Term of S,V;

theorem Th21:
  for o being OperSymbol of S, a being FinSequence of S-Terms V
  holds a is ArgumentSeq of Sym(o,V) iff Sym(o, V) ==> roots a
proof
  let o be OperSymbol of S, a be FinSequence of S-Terms V;
  a is SubtreeSeq of Sym(o, V) iff Sym(o, V) ==> roots a by DTCONSTR:def 6;
  hence thesis by Def2;
end;
