The rules that use the category "<id_letter-sing>" are:

<bare_company_id>

<company_noun-sing> <id_letter-sing>

<id_letter-sing> <company_noun-sing>

<company_id-not_creatable>

<company_type-sing> <id_letter-sing>

<id_letter-sing> <company_noun-sing>

<bare_company_id-sing>

<company_noun-sing> <id_letter-sing>

<id_letter-sing> <company_noun-sing>

<company_id-not_creatable-sing>

<company_type-sing> <id_letter-sing>

<id_letter-sing> <company_noun-sing>

<company_id-creatable-sing>

<company_type-sing> <id_letter-sing>

<id_letter-sing> <company_noun-sing>