The algebraic expressions produced by the translation in Section 18.2 Translation to the SPARQL Algebra may contain, as a sub-expression, a multiset of solution mappings or a sequence of solution mappings. In particular,

• a multiset of solution mappings becomes a sub-expression if the WHERE clause of the given query contains a VALUES clause: see the point "If the form is InlineData" in Section 18.2.2.6 Translate Graph Patterns,
• a sequence of solution mappings becomes a sub-expression if the given query contains a trailing VALUES clause: see Section 18.2.4.3 VALUES.

Yet, the eval function, which defines the evaluation semantics of the algebraic expressions, is undefined for these two cases. That is, Section 18.5.2 Evaluation Semantics does not cover the case of eval(D(G), A) with A being a multiset of solution mappings, nor the case in which A is a sequence of solution mappings.

Of course, it is not difficult to guess how eval is supposed to be defined for these two cases, but for the sake of formal completeness, these cases should be covered explicitly in the definition of eval.