gistfile1.txt

let
  overlayA = self: super: {
    python27 = super.python27.override {
      packageOverrides = python-self: python-super: {
        foo = python-super.twisted;
      };
    };
  };
  packageOverridesB = python-self: python-super: {
    bar = python-super.foo;
  };
  overlayB = self: super: {
    python27 = super.python27.override (old: {
      packageOverrides = super.lib.composeExtensions old.packageOverrides packageOverridesB;
    });
  };
in
with import <nixpkgs> {
  overlays = [
    overlayA
    overlayB
  ];
};
assert python27.pkgs.bar == python27.pkgs.twisted; 1