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[Anders Logg <logg@xxxxxxxxx>]

After Marie's latest addition of enriched spaces (and some discussion
with Doug Arnold), it seems clear that our current notation V + W for
mixed spaces is not optimal.

Even though one may think of the operation of creating a "mixed
function space" as a direct sum,

  X = {(v, 0) : v in V} \oplus {(0, w) : w in W},

it is more natural (and common) to think of it as a Cartesian product,

  X = V \times W = {(v, w) : v in V, w in W}

It would therefore be more natural to use '*' instead of '+' as the
operation for creating mixed elements/function spaces.

That would free up '+' to be used for enriched spaces (which have
recently been added),

  X = {v + w : v in V, w in W}

The typical example would be to take V piecewise linears and W scaled
P3 bubbles.

In summary, the suggestion is to use the following notation:

  +  <-->  +
  *  <-->  \times

It's obvious this is better than what we have now which is

  +  <--> \oplus
  ?  <--> +

Thoughts?

--
Anders

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