* Added a primop `removeAttrs' to remove attributes from a set, e.g.,

`removeAttrs attrs ["x", "y"]' returns the set `attrs' with the
  attributes named `x' and `y' removed.  It is not an error for the
  named attributes to be missing from the input set.

tests/lang/eval-fail-remove.nix

@@ -0,0 +1,5 @@
+let {
+  attrs = {x = 123; y = 456;};
+
+  body = (removeAttrs attrs ["x"]).x;
+}

tests/lang/eval-okay-remove.exp

@@ -0,0 +1 @@
+Int(456)

tests/lang/eval-okay-remove.nix

@@ -0,0 +1,5 @@
+let {
+  attrs = {x = 123; y = 456;};
+
+  body = (removeAttrs attrs ["x"]).y;
+}