public function setFirst($first)
    {
        if (Format::isIdentifier($first)) {
            $this->first = $first;
        } else {
            $msg = "'${first}' is an invalid QTI identifier.";
            throw new InvalidArgumentException($msg);
        }
    }
public function setFirst($first)
    {
        if (Format::isIdentifier($first)) {
            $this->first = $first;
        } else {
            $msg = "'${first}' is an invalid QTI identifier.";
            throw new InvalidArgumentException($msg);
        }
    }
public function setFirst ( $ first ) { if ( Format :: isIdentifier ( $ first ) ) { $ this -> first = $ first ; } else { $ msg = "'${first}' is an invalid QTI identifier." ; throw new InvalidArgumentException ( $ msg ) ; } }
Set the first identifier of the pair.

@param string $first A QTI Identifier.
@throws \InvalidArgumentException If $first is an invalid QTI Identifier.
Set the first identifier of the pair .