public function concat($expression1, $expression2, ...$expressions) : self
    {
        $this->expr->concat(...func_get_args());

        return $this;
    }
public function concat($expression1, $expression2, ...$expressions) : self
    {
        $this->expr->concat(...func_get_args());

        return $this;
    }
public function concat ( $ expression1 , $ expression2 , ... $ expressions ) : self { $ this -> expr -> concat ( ... func_get_args ( ) ) ; return $ this ; }
Concatenates strings and returns the concatenated string.

The arguments can be any valid expression as long as they resolve to
strings. If the argument resolves to a value of null or refers to a field
that is missing, $concat returns null.

@see http://docs.mongodb.org/manual/reference/operator/aggregation/concat/
@see Expr::concat

@param mixed|Expr $expression1
@param mixed|Expr $expression2
@param mixed|Expr ...$expressions Additional expressions
Concatenates strings and returns the concatenated string .