theorem Th21:
  for S being non empty non void ManySortedSign
  for o being OperSymbol of S st the_arity_of o = {}
  for A being MSAlgebra over S holds Args(o,A) = {{}}
  proof
    let S be non empty non void ManySortedSign;
    let o be OperSymbol of S;
    assume A1: the_arity_of o = {};
    let A be MSAlgebra over S;
    thus Args(o,A) = product ((the Sorts of A)*the_arity_of o) by PRALG_2:3
    .= {{}} by A1,CARD_3:10;
  end;
