title: How many possible combinations can i get from this hash?
body: <p>Here is a the combination:</p>
  
  <pre>
  OPTIONS = {
    :tense =>     [:present,    :past,       :future],
    :person =>    [:third,      :first,      :second],
    :plurality => [:singular,   :plural],
    :diathesis => [:active,     :passive],
    :mood =>      [:indicative, :imperative, :subjunctive],
    :aspect =>    [:habitual,   :perfective, :habitual, :progressive, :perfect, :prospective]  
  }
  </pre>
  
  <p>There is only one rule, one value can be used from one key</p>
  
  <p>so that means these pairs are possible</p>
  
  <pre>
  1. :present, :third, :singular, :active, :indicative, :habitual
  2. :present, :third
  3. :present, :third, :indicative, :habitual 
  4. ... etc
  </pre>
  
  <p>however these ones are not:</p>
  
  <pre>
  1. :present, :past, :future
  2. :first, :third, :present
  3. ... etc
  </pre>
  
  <p>If there is a formula please share</p>
  
  <p>All answers are appreciated.</p>