ASN.1 Standart

Hi,

I've idea to implement ASN.1 language on MPS and I've just begun to learn MPS and have a little expirience in one. In standart sometimes meets expressions like

SequenceType ::=
SEQUENCE "{" "}"
| SEQUENCE "{" ExtensionAndException OptionalExtensionMarker "}"
| SEQUENCE "{" ComponentTypeLists "}"

It means that SequenceType maybe presented in three variants, there are:

1. SEQUENCE { }

2. SEQUENCE { ExtensionAndException OptionalExtensionMarker}

3. SEQUENCE { ComponentTypeLists }

I see only one solution - for each variant create concept which implements some common base interface.

Can you give me more proper solution for this problem? Or my solution is enough properly?