* Make printing an expression as XML interruptible.

src/libexpr/expr-to-xml.cc

@@ -2,6 +2,7 @@
 #include "xml-writer.hh"
 #include "nixexpr-ast.hh"
 #include "aterm.hh"
+#include "util.hh"
 
 
 namespace nix {
@@ -24,6 +25,8 @@ static void printTermAsXML(Expr e, XMLWriter & doc, PathSet & context)
     ATermList as, es, formals;
     ATerm body, pos;
 
+    checkInterrupt();
+
     if (matchStr(e, s, context)) /* !!! show the context? */
         doc.writeEmptyElement("string", singletonAttrs("value", s));