I have come across a dash-spacing style that I haven't seen anywhere else, and I'm wondering where it may come from. The person who writes in this style asserts that he has learnt it in school in the UK.
Briefly on dash styles: Style manuals vary in how they recommend using dashes to separate interpolated phrases, the two most common appearing to be the spaced en-dash and the unspaced em-dash (see Wikipedia article Dash, and the questions Is it wrong to space en dashes and em dashes? and When should I use an em-dash, an en-dash, and a hyphen? on this site).
But the style I have come across is different and seems to have the following rules that are a mix of other rules I have seen:
When a phrase is interpolated so that the main sentence continues, it is set off by the sequence "space, en-dash" in the beginning, and "en-dash, space" at the end.
Example: This sentence –being an example– makes no sense.
When a phrase is interpolated at the end of the main sentence, it is set off by the sequence "space, en-dash, space".
Example: This sentence makes no sense – it is an example.
Has anyone come across this style before? Where could it originate from? Is the rule set above complete and correct for this style, and is there any rationale for the style?