diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 802b83aa160dd7686ad9aa4f85154002355998b7..eaa4b4ea444c74c32971f86964cb1e21c380144f 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -134,6 +134,18 @@ ATerm expandRec(ATerm e, ATermList rbnds, ATermList nrbnds)
 }
 
 
+static Expr updateAttrs(Expr e1, Expr e2)
+{
+    /* Note: e1 and e2 should be in normal form. */
+
+    ATermMap attrs;
+    queryAllAttrs(e1, attrs);
+    queryAllAttrs(e2, attrs);
+
+    return makeAttrs(attrs);
+}
+
+
 string evalString(EvalState & state, Expr e)
 {
     e = evalExpr(state, e);
@@ -264,6 +276,10 @@ Expr evalExpr2(EvalState & state, Expr e)
     if (atMatch(m, e) >> "OpOr" >> e1 >> e2)
         return makeBool(evalBool(state, e1) || evalBool(state, e2));
 
+    /* Attribut set update (//). */
+    if (atMatch(m, e) >> "OpUpdate" >> e1 >> e2)
+        return updateAttrs(evalExpr(state, e1), evalExpr(state, e2));
+
     /* Barf. */
     throw badTerm("invalid expression", e);
 }
diff --git a/src/libexpr/lexer.l b/src/libexpr/lexer.l
index 853362cd0bbb064aa0a3dd02f876a5cff93e237c..ce1c4673d7a603debe439ee37d3873a18be7abdc 100644
--- a/src/libexpr/lexer.l
+++ b/src/libexpr/lexer.l
@@ -57,6 +57,7 @@ inherit     { return INHERIT; }
 \&\&        { return AND; }
 \|\|        { return OR; }
 \-\>        { return IMPL; }
+\/\/        { return UPDATE; }
 
 {ID}        { yylval->t = ATmake("<str>", yytext); return ID; /* !!! alloc */ }
 {INT}       { int n = atoi(yytext); /* !!! overflow */
diff --git a/src/libexpr/parser.y b/src/libexpr/parser.y
index d97106fcae67b2c0982e0cb7c515800c8c68051c..6c0fdbda25f32ce2f4733b1af1f4e6b3f2d60310 100644
--- a/src/libexpr/parser.y
+++ b/src/libexpr/parser.y
@@ -42,6 +42,7 @@ void yyerror(YYLTYPE * loc, yyscan_t scanner, void * data, char * s)
 %left OR
 %left AND
 %nonassoc EQ NEQ
+%right UPDATE
 %left NEG
 
 %%
@@ -69,6 +70,7 @@ expr_op
   | expr_op AND expr_op { $$ = ATmake("OpAnd(<term>, <term>)", $1, $3); }
   | expr_op OR expr_op { $$ = ATmake("OpOr(<term>, <term>)", $1, $3); }
   | expr_op IMPL expr_op { $$ = ATmake("OpImpl(<term>, <term>)", $1, $3); }
+  | expr_op UPDATE expr_op { $$ = ATmake("OpUpdate(<term>, <term>)", $1, $3); }
   | expr_app
   ;