* New builtin functions builtins.{hasAttr, getAttr} to check for

attribute existence and to return an attribute from an attribute
  set, respectively.  Example: `hasAttr "foo" {foo = 1;}'.  They
  differ from the `?' and `.' operators in that the attribute name is
  an arbitrary expression.  (NIX-61)

tests/lang/eval-okay-attrs2.nix

@@ -0,0 +1,10 @@
+let {
+  as = { x = 123; y = 456; } // { z = 789; } // { z = 987; };
+
+  A = "a";
+  Z = "z";
+
+  body = if builtins.hasAttr A as
+         then builtins.getAttr A as
+         else assert builtins.hasAttr Z as; builtins.getAttr Z as;
+}