```ruby
module Siff
  
  SIFFS = {
    :BE => 'EUR',
    :BG => 'EUR',
    :CA => 'CAD',
    :CZ => 'EUR',
    :DK => 'EUR',
    :DE => 'EUR',
    :EE => 'EUR',
    :IE => 'EUR',
    :EL => 'EUR',
    :ES => 'EUR',
    :FR => 'EUR',
    :GB => 'GBP',
    :IT => 'EUR',
    :CY => 'EUR',
    :LV => 'EUR',
    :LT => 'EUR',
    :LU => 'EUR',
    :HU => 'EUR',
    :MT => 'EUR',
    :NL => 'EUR',
    :AT => 'EUR',
    :PL => 'EUR',
    :PT => 'EUR',
    :RO => 'EUR',
    :SI => 'EUR',
    :SK => 'EUR',
    :FI => 'EUR',
    :SE => 'EUR',
    :UK => 'GBP',
    :US => 'USD'
  }
  def currency_for(country_code)
    SIFFS[country_code] || 'unknown'
  end
end
```

Convert strings to symbols to save memory instantiation.
user
```ruby
module Siff
  
  SIFFS = {
    :BE => :EUR,
    :BG => :EUR,
    :CA => :CAD,
    :CZ => :EUR,
    :DK => :EUR,
    :DE => :EUR,
    :EE => :EUR,
    :IE => :EUR,
    :EL => :EUR,
    :ES => :EUR,
    :FR => :EUR,
    :GB => :GBP,
    :IT => :EUR,
    :CY => :EUR,
    :LV => :EUR,
    :LT => :EUR,
    :LU => :EUR,
    :HU => :EUR,
    :MT => :EUR,
    :NL => :EUR,
    :AT => :EUR,
    :PL => :EUR,
    :PT => :EUR,
    :RO => :EUR,
    :SI => :EUR,
    :SK => :EUR,
    :FI => :EUR,
    :SE => :EUR,
    :UK => :GBP,
    :US => :USD
  }
  def currency_for(country_code)
    (SIFFS[country_code] || :unknown).to_s
  end
end
```
assistant