code: public Formula naryOperator(final FType type, final Collection<? extends Formula> operands) {
      return this.naryOperator(type, operands.toArray(new Formula[0]));
    }
docs: Creates a new n-ary operator with a given type and a list of operands.
  @param type     the type of the formula
  @param operands the list of operands
  @return the newly generated formula
  @throws IllegalArgumentException if a wrong formula type is passed