reserve i,j for Nat;
reserve i,j for Nat,
  x for variable,
  l for quasi-loci;
reserve C for initialized ConstructorSignature,
  c for constructor OperSymbol of C;

theorem Th33:
  for C for e being expression of C holds e is subexpression of e
  proof
    let C be initialized ConstructorSignature;
    let e be expression of C;
    thus e in Subtrees e by TREES_9:11;
  end;
