Support index_set on expressions other than identifiers
Support index_set on expressions other than identifiers

jjdekker/libminizinc,tias/libminizinc,nathanielbaxter/libminizinc,jjdekker/libminizinc,tias/libminizinc,nathanielbaxter/libminizinc,jjdekker/libminizinc,jjdekker/libminizinc,nathanielbaxter/libminizinc,nathanielbaxter/libminizinc