removing rows from a table : specifying row index

removing rows from a table : specifying row index

Hi All, 
This is having me puzzled. 

I have table with id = "rules" and <tbody> tag defined. I am trying to delete all the rows except first two rows in <tbody> .

$('#rules > tbody > tr:gt(1)').remove();  //this works (if I assign  '1' directly in command) 

However, the following code doesn't work. 

var a = 1;
$('#rules > tbody > tr:gt( a)').remove(); //doesn't work . 

I tried different approaches, and in all cases if I assign a integer value as index , it works. However, if I assign a variable for index, it doesn't work. 

Can you help me please?