code: function dice( source, target ) {
    
    var sources = dice.prepare( source )
    var targets = dice.prepare( target )
    
    var i, k, intersection = 0
    var source_count = sources.length
    var target_count = targets.length
    var union = source_count + target_count
    
    for( i = 0; i < source_count; i++ ) {
      source = sources[i]
      for( k = 0; k < target_count; k++ ) {
        target = targets[k]
        if( source === target ) {
          intersection++
          targets[k] = undefined
          break
        }
      }
    }
    
    return 2 * intersection / union
    
  }
docs: Computes dice's coefficient.
  
  @param  {String} source
  @param  {String} target
  @return {Number} dice coefficient