Edit definitions in the 'SPARQL Algebra' section

[hartig]

As @TallTed suggests (here, here, and here), the definitions in Section 18.6 SPARQL Algebra should be edited as follows:

1. The We define: part in each of these definitions should be extended to We define the following:
2. The block of formulas in each of the definitions should be represented as a list (unorded).

This should be done after PR #255 is merged.

[afs]

The block of formulas in each of the definitions should be represented as a list (unordered).

Lists have bullet or number markers. I don't see that working. Formal definitions are not written that way. Indent yes; markers, no; ideally for me, with a slightly different background to show the definition as a unit.

Probably something to do via cleaning up styling and fixing the CSS.

[TallTed]

The change is from We define: to We define the following:. This is because the former improperly ends in a colon, which should only go where a sentence concludes -- i.e., where a period could be used instead -- but We define is not a complete sentence. The replacement could be improved, as We define the following <noun(s)>:

It's fine for there to be one (or more) item after the following [<collective_noun(s)>]:, e.g., We define the following rule(s):. I'm not sure what the generic collective_noun is in these cases, so I didn't suggest one. Probably @hartig or maybe @afs can supply the collective_noun.

(Better to just have the relevant plural or singular noun, e.g., collective_noune, than to have the singular-or-plural, e.g., collective_noun(s), but I'm writing a generic here, so I'm using the flexible word.)