Confusing Use of 'fmap' in solrCell Command Example

The following is an excerpt from the example within the documentation of solrCell:

  # rename "content" field to "text" fields
  fmap : { content : text, content-type : content_type }

  lowernames : true

I find this example confusing because it doesn't seem that there's any need to include the content-type field in fmap. While I expect that the intention was to replace the illegal '-' character in the Solr field name with an underscore, that was not explained in the comment. According to the description of 'lowernames' in the documentation that precedes this example, 'lowernames' will "map all field names to lowercase with underscores," so doing this in 'fmap' does not seem necessary.