gistfile1.txt

let
  overlayA = self: super: {                                                                                                                                                                                                                                                   
    python27 = super.python27.override {                                                                                                                                                                                                                                      
      packageOverrides = python-self: python-super: {                                                                                                                                                                                                                         
        foo = "foo";
      };
    };
  };
  packageOverridesB = python-self: python-super: {                                                                                                                                                                                                                            
    foo = python-super.foo + "bar";                                                                                                                                                                                                                                           
  };
  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.foo == "foobar"; 1