class Haml::Parser

def merge_value(key, to, from)
  if from.kind_of?(Hash) || to.kind_of?(Hash)
    from = { nil => from } if !from.is_a?(Hash)
    to   = { nil => to }   if !to.is_a?(Hash)
    to.merge(from)
  elsif key == 'id'
    merged_id = filter_and_join(from, '_')
    if to && merged_id
      merged_id = "#{to}_#{merged_id}"
    elsif to || merged_id
      merged_id ||= to
    end
    merged_id
  elsif key == 'class'
    merged_class = filter_and_join(from, ' ')
    if to && merged_class
      merged_class = (to.split(' ') | merged_class.split(' ')).join(' ')
    elsif to || merged_class
      merged_class ||= to
    end
    merged_class
  else
    from
  end
end