convert EBNF to BNF loses optional token?

Given this input:
```
filedecl_beg = KW_FILEDECL_BEG, STRING, [ STRING ] ;
```
I run kgt with this command:
```
./kgt -l iso-ebnf -e bnf <input.ebnf
```
Actual output:
```
<filedecl_beg> ::= <KW_FILEDECL_BEG> <STRING>
```
Expected output, I cannot present here, but would expect that the second, optional `STRING` is present in the BNF output.