code: public void setErr(String errprop) {
      this.errprop = errprop;
      err = (errprop.equals(outprop)) ? err = out : new ByteArrayOutputStream();
    }
docs: Sets the property into which System.err will be written. If this property
  has the same name as the property for System.out, the two will be interlaced.
  @param errprop The name of the property into which System.err is written.
  If the property is defined before this task is executed, it will not be updated.