[r-t] Double Grandsire Minor

[Alexander Holroyd]

One small clarification here: Since Q-sets have 4 members, even adding one 
Q-set changes the parity of the number of round blocks.  Parity per se is 
not the issue.  Richard's elegant 2 Q-set linkage is perhaps more akin to 
the twin bob linkage of Stedman Triples.

On Sat, 16 Feb 2013, Richard Pullin wrote:

> Due to the presence of half-lead calls, it is easy to select an omit Q-set
> at the lead-end which mirrors an identical Q-set at the half-lead.
> This effectively splits one round block into two, meaning we can easily
> switch the parity of the total number of blocks without adding any extra
> material (sort of similar to the magic blocks in bobs-only Stedman Triples).
> By using this technique, the extent formed itself out of four mutually true
> detached blocks, and a 'keystone' Q-set divided over the four blocks was
> found to finish it off into a 720.