A BNF grammar that is equivalent to the following EBNF grammar

$[e|]$

is

$SxfY$

$xc|e|$

$Y1$

What should go in the empty slot? Separate symbols in your answer by exactly ONE space. For example, if the answer is a $x,x,$ you should write axx. No extra spaces in the middle.

The correct answer is: aby