Add paragraph on interaction between delaying and aggregation

chapters/2_background.tex

@@ -1901,6 +1901,17 @@ Note that this heuristic does not guarantee that \variables{} have their
 tightest possible \gls{domain}. One delayed \constraint{} can still influence
 the \glspl{domain} of \variables{} used by other delayed \constraints{}.
 
+Delaying the rewriting of constraints might, however, interfere with the
+constraint aggregation. Since aggregation is eagerly performed only when a
+\constraint{} is first encountered, it cannot aggregate any constraints that
+follow from delayed values. For example, if when aggregating Boolean clauses
+comes across an expression that needs to be reified, then a new Boolean
+\variable{} is created and the reified \constraint{} is delayed. The problem
+is, however, that if the definition of this reified constraint turn out to be
+in terms of Boolean clauses as well, then this definition could have been
+aggregated as well. Because the flattener does not revisit the aggregation of
+variables, this does not happen.
+
 \subsection{FlatZinc Optimisation}%
 \label{subsec:back-fzn-optimisation}