public function subtract($expression1, $expression2) : self
    {
        $this->expr->subtract($expression1, $expression2);

        return $this;
    }
public function subtract($expression1, $expression2) : self
    {
        $this->expr->subtract($expression1, $expression2);

        return $this;
    }
public function subtract ( $ expression1 , $ expression2 ) : self { $ this -> expr -> subtract ( $ expression1 , $ expression2 ) ; return $ this ; }
Subtracts two numbers to return the difference. The second argument is
subtracted from the first argument.

The arguments can be any valid expression as long as they resolve to numbers and/or dates.

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

@param mixed|Expr $expression1
@param mixed|Expr $expression2
Subtracts two numbers to return the difference . The second argument is subtracted from the first argument .