Symbol L in 'Evaluation Semantics' section should not be a solution sequence

[hartig]

Section 18.6.2 Evaluation Semantics introduces the symbol L to be a solution sequence (see the three bullet points at the beginning of the section). Yet, the way this symbol is used later in the section, it is not a solution sequence but an algebraic query expression used as a subexpression for the cases in which the evaluation produces a solution sequence.

A fix is to simply change the bullet point that introduces L --- changing it from:

• L : a solution sequence

to:

• L : an algebraic query expression

[hartig]

I notice that the symbols P, P₁, and P₂ are introduced as "graph patterns", which is also wrong. They should also be algebraic query expressions. Consequently, the bullet point for them should also be fixed, and they should be renamed to A', A₁, and A₂.

After this change, the symbol L can be dropped completely and A' can be used in the definitions that currently use L.