public function avg($expression1, ...$expressions) : self
    {
        $this->expr->avg(empty($expressions) ? $expression1 : func_get_args());

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

        return $this;
    }
public function avg ( $ expression1 , ... $ expressions ) : self { $ this -> expr -> avg ( empty ( $ expressions ) ? $ expression1 : func_get_args ( ) ) ; return $ this ; }
Returns the average value of the numeric values that result from applying
a specified expression to each document in a group of documents that
share the same group by key. Ignores nun-numeric values.

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

@param mixed|Expr $expression1
@param mixed|Expr ...$expressions Additional expressions
Returns the average value of the numeric values that result from applying a specified expression to each document in a group of documents that share the same group by key . Ignores nun - numeric values .