Aggregation definition: states that it returns a single partial function

Closes #107

spec/index.html

@@ -8894,7 +8894,7 @@ <h5>Grouping and Aggregation</h5>
       Replace V with Sample(V)
       End
   For each aggregate R(args ; scalarvals) now in X
-      # note scalarvals may be omitted, then it's equivalent to the empty set
+      # note scalarvals may be omitted, then it's equivalent to the empty function
       A<span><sub>i</sub></span> := Aggregation(args, R, scalarvals, Grp)
       Replace R(...) with agg<span><sub>i</sub></span> in Q
       i := i + 1
@@ -9555,9 +9555,9 @@ <h4>Aggregate Algebra</h4>
               <b>Definition: Aggregation</b>
             </div>
             <p>Let <i>exprlist</i> be a list of expressions or *, <i>func</i> a set function,
-              <i>scalarvals</i> a set of partial functions (possibly empty) passed from the aggregate
+              <i>scalarvals</i> a partial function (possibly empty) passed from the aggregate
               in the query, and let { key<sub>1</sub>→Ψ<sub>1</sub>, ...,
-              key<sub>m</sub>→Ψ<sub>m</sub> } be a set of partial functions from keys to
+              key<sub>m</sub>→Ψ<sub>m</sub> } be a partial function from keys to
               solution sequences as produced by the grouping step.</p>
             <p>Aggregation applies the set function func to the given set and produces a
               single value for each key and group of solutions for that key.</p>