diff --git a/src/eval.cc b/src/eval.cc
index 4eb222197d..be8b70a039 100644
--- a/src/eval.cc
+++ b/src/eval.cc
@@ -106,7 +106,7 @@ static void runProgram(const string & program, Environment env)
             
         } catch (exception & e) {
             cerr << format("build error: %1%\n") % e.what();
-        }
+         }
         _exit(1);
 
     }
@@ -159,38 +159,6 @@ Hash hashTerm(ATerm t)
 }
 
 
-#if 0
-/* Evaluate a list of arguments into normal form. */
-void evalArgs(ATermList args, ATermList & argsNF, Environment & env)
-{
-    argsNF = ATempty;
-
-    while (!ATisEmpty(args)) {
-        ATerm eName, eVal, arg = ATgetFirst(args);
-        if (!ATmatch(arg, "Tup(<term>, <term>)", &eName, &eVal))
-            throw badTerm("invalid argument", arg);
-
-        string name = evalString(eName);
-        eVal = evalValue(eVal);
-
-        char * s;
-        if (ATmatch(eVal, "Str(<str>)", &s)) {
-            env[name] = s;
-        } else if (ATmatch(eVal, "Hash(<str>)", &s)) {
-            env[name] = queryValuePath(parseHash(s));
-        } else throw badTerm("invalid argument value", eVal);
-
-        argsNF = ATinsert(argsNF,
-            ATmake("Tup(Str(<str>), <term>)", name.c_str(), eVal));
-
-        args = ATgetNext(args);
-    }
-
-    argsNF = ATreverse(argsNF);
-}
-#endif
-
-
 struct RStatus
 {
     /* !!! the comparator of this hash should match the semantics of
@@ -362,7 +330,7 @@ static FState realise(RStatus & status, FState fs)
         return nf;
     }
 
-    throw badTerm("bad file system state expression", fs);
+    throw badTerm("bad fstate expression", fs);
 }
 
 
diff --git a/src/fix.cc b/src/fix.cc
deleted file mode 100644
index 3c4c8bf539..0000000000
--- a/src/fix.cc
+++ /dev/null
@@ -1,368 +0,0 @@
-#include <iostream>
-#include <map>
-
-#include <sys/types.h>
-#include <sys/stat.h>
-#include <sys/wait.h>
-
-extern "C" {
-#include <aterm2.h>
-}
-
-#include "util.hh"
-#include "hash.hh"
-
-
-static string nixDescriptorDir;
-
-
-static bool verbose = false;
-
-
-/* Mapping of Fix file names to the hashes of the resulting Nix
-   descriptors. */
-typedef map<string, Hash> DescriptorMap;
-
-
-void registerFile(string filename)
-{
-    int res = system(("nix regfile " + filename).c_str()); 
-    /* !!! escape */
-    if (WEXITSTATUS(res) != 0)
-        throw Error("cannot register " + filename + " with Nix");
-}
-
-
-void registerURL(Hash hash, string url)
-{
-    int res = system(("nix regurl " + (string) hash + " " + url).c_str());
-    /* !!! escape */
-    if (WEXITSTATUS(res) != 0)
-        throw Error("cannot register " + 
-            (string) hash + " -> " + url + " with Nix");
-}
-
-
-Error badTerm(const string & msg, ATerm e)
-{
-    char * s = ATwriteToString(e);
-    return Error(msg + ", in `" + s + "'");
-}
-
-
-/* Term evaluation. */
-
-typedef map<string, ATerm> BindingsMap;
-
-struct EvalContext
-{
-    string dir;
-    DescriptorMap * done;
-    BindingsMap * vars;
-};
-
-
-ATerm evaluate(ATerm e, EvalContext ctx);
-Hash instantiateDescriptor(string filename, EvalContext ctx);
-
-
-string evaluateStr(ATerm e, EvalContext ctx)
-{
-    e = evaluate(e, ctx);
-    char * s;
-    if (ATmatch(e, "Str(<str>)", &s))
-        return s;
-    else throw badTerm("string value expected", e);
-}
-
-
-bool evaluateBool(ATerm e, EvalContext ctx)
-{
-    e = evaluate(e, ctx);
-    if (ATmatch(e, "Bool(True)"))
-        return true;
-    else if (ATmatch(e, "Bool(False)"))
-        return false;
-    else throw badTerm("boolean value expected", e);
-}
-
-
-ATerm evaluate(ATerm e, EvalContext ctx)
-{
-    char * s;
-    ATerm e2, e3;
-    ATerm eCond, eTrue, eFalse;
-
-    /* Check for normal forms first. */
-
-    if (ATmatch(e, "Str(<str>)", &s) ||
-        ATmatch(e, "Bool(True)") || ATmatch(e, "Bool(False)"))
-        return e;
-    
-    else if (
-        ATmatch(e, "Pkg(<str>)", &s) || 
-        ATmatch(e, "File(<str>)", &s))
-    {
-        parseHash(s);
-        return e;
-    }
-
-    /* Short-hands. */
-
-    else if (ATmatch(e, "<str>", &s))
-        return ATmake("Str(<str>)", s);
-
-    else if (ATmatch(e, "True", &s))
-        return ATmake("Bool(True)", s);
-
-    else if (ATmatch(e, "False", &s))
-        return ATmake("Bool(False)", s);
-
-    /* Functions. */
-
-    /* `Var' looks up a variable. */
-    else if (ATmatch(e, "Var(<str>)", &s)) {
-        string name(s);
-        ATerm e2 = (*ctx.vars)[name];
-        if (!e2) throw Error("undefined variable " + name);
-        return evaluate(e2, ctx); /* !!! update binding */
-    }
-
-    /* `Fix' recursively instantiates a Fix descriptor, returning the
-       hash of the generated Nix descriptor. */
-    else if (ATmatch(e, "Fix(<term>)", &e2)) {
-        string filename = absPath(evaluateStr(e2, ctx), ctx.dir); /* !!! */
-        return ATmake("Pkg(<str>)",
-            ((string) instantiateDescriptor(filename, ctx)).c_str());
-    }
-
-#if 0
-    /* `Source' copies the specified file to nixSourcesDir, registers
-       it with Nix, and returns the hash of the file. */
-    else if (ATmatch(e, "Source(<term>)", &e2)) {
-        string source = absPath(evaluateStr(e2, ctx), ctx.dir); /* !!! */
-        string target = nixSourcesDir + "/" + baseNameOf(source);
-
-        // Don't copy if filename is already in nixSourcesDir.
-        if (source != target) {
-            if (verbose)
-                cerr << "copying source " << source << endl;
-            string cmd = "cp -p " + source + " " + target;
-            int res = system(cmd.c_str());
-            if (WEXITSTATUS(res) != 0)
-                throw Error("cannot copy " + source + " to " + target);
-        }
-        
-        registerFile(target);
-        return ATmake("File(<str>)", hashFile(target).c_str());
-    }
-#endif
-
-    /* `Local' registers a file with Nix, and returns the file's
-       hash. */
-    else if (ATmatch(e, "Local(<term>)", &e2)) {
-        string filename = absPath(evaluateStr(e2, ctx), ctx.dir); /* !!! */
-        Hash hash = hashFile(filename);
-        registerFile(filename); /* !!! */
-        return ATmake("File(<str>)", ((string) hash).c_str());
-    }
-
-    /* `Url' registers a mapping from a hash to an url with Nix, and
-       returns the hash. */
-    else if (ATmatch(e, "Url(<term>, <term>)", &e2, &e3)) {
-        Hash hash = parseHash(evaluateStr(e2, ctx));
-        string url = evaluateStr(e3, ctx);
-        registerURL(hash, url);
-        return ATmake("File(<str>)", ((string) hash).c_str());
-    }
-
-    /* `If' provides conditional evaluation. */
-    else if (ATmatch(e, "If(<term>, <term>, <term>)", 
-                 &eCond, &eTrue, &eFalse)) 
-        return evaluate(evaluateBool(eCond, ctx) ? eTrue : eFalse, ctx);
-
-    else throw badTerm("invalid expression", e);
-}
-
-
-string getStringFromMap(BindingsMap & bindingsMap,
-    const string & name)
-{
-    ATerm e = bindingsMap[name];
-    if (!e) throw Error("binding " + name + " is not set");
-    char * s;
-    if (ATmatch(e, "Str(<str>)", &s))
-        return s;
-    else
-        throw Error("binding " + name + " is not a string");
-}
-
-
-/* Instantiate a Fix descriptors into a Nix descriptor, recursively
-   instantiating referenced descriptors as well. */
-Hash instantiateDescriptor(string filename, EvalContext ctx)
-{
-    /* Already done? */
-    DescriptorMap::iterator isInMap = ctx.done->find(filename);
-    if (isInMap != ctx.done->end()) return isInMap->second;
-
-    /* No. */
-    ctx.dir = dirOf(filename);
-
-    /* Read the Fix descriptor as an ATerm. */
-    ATerm inTerm = ATreadFromNamedFile(filename.c_str());
-    if (!inTerm) throw Error("cannot read aterm " + filename);
-
-    ATerm bindings;
-    if (!ATmatch(inTerm, "Descr(<term>)", &bindings))
-        throw Error("invalid term in " + filename);
-    
-    /* Iterate over the bindings and evaluate them to normal form. */
-    BindingsMap bindingsMap; /* the normal forms */
-    ctx.vars = &bindingsMap;
-
-    char * cname;
-    ATerm value;
-    while (ATmatch(bindings, "[Bind(<str>, <term>), <list>]", 
-               &cname, &value, &bindings)) 
-    {
-        string name(cname);
-        ATerm e = evaluate(value, ctx);
-        bindingsMap[name] = e;
-    }
-
-    /* Construct a descriptor identifier by concatenating the package
-       and release ids. */
-    string pkgId = getStringFromMap(bindingsMap, "pkgId");
-    string releaseId = getStringFromMap(bindingsMap, "releaseId");
-    string id = pkgId + "-" + releaseId;
-    bindingsMap["id"] = ATmake("Str(<str>)", id.c_str());
-
-    /* Add a system name. */
-    bindingsMap["system"] = ATmake("Str(<str>)", thisSystem.c_str());
-         
-    /* Construct the resulting ATerm.  Note that iterating over the
-       map yields the bindings in sorted order, which is exactly the
-       canonical form for Nix descriptors. */
-    ATermList bindingsList = ATempty;
-    for (BindingsMap::iterator it = bindingsMap.begin();
-         it != bindingsMap.end(); it++)
-        /* !!! O(n^2) */
-        bindingsList = ATappend(bindingsList,
-            ATmake("Bind(<str>, <term>)", it->first.c_str(), it->second));
-    ATerm outTerm = ATmake("Descr(<term>)", bindingsList);
-
-    /* Write out the resulting ATerm. */
-    string tmpFilename = nixDescriptorDir + "/tmp";
-    if (!ATwriteToNamedTextFile(outTerm, tmpFilename.c_str()))
-        throw Error("cannot write aterm to " + tmpFilename);
-
-    Hash outHash = hashFile(tmpFilename);
-    string outFilename = nixDescriptorDir + "/" + 
-        id + "-" + (string) outHash + ".nix";
-    if (rename(tmpFilename.c_str(), outFilename.c_str()))
-        throw Error("cannot rename " + tmpFilename + " to " + outFilename);
-
-    /* Register it with Nix. */
-    registerFile(outFilename);
-
-    if (verbose)
-        cerr << "instantiated " << (string) outHash 
-             << " from " << filename << endl;
-
-    (*ctx.done)[filename] = outHash;
-    return outHash;
-}
-
-
-/* Instantiate a set of Fix descriptors into Nix descriptors. */
-void instantiateDescriptors(Strings filenames)
-{
-    DescriptorMap done;
-
-    EvalContext ctx;
-    ctx.done = &done;
-
-    for (Strings::iterator it = filenames.begin();
-         it != filenames.end(); it++)
-    {
-        string filename = absPath(*it);
-        cout << (string) instantiateDescriptor(filename, ctx) << endl;
-    }
-}
-
-
-/* Print help. */
-void printUsage()
-{
-    cerr <<
-"Usage: fix ...\n\
-";
-}
-
-
-/* Parse the command-line arguments, call the right operation. */
-void run(Strings::iterator argCur, Strings::iterator argEnd)
-{
-    umask(0022);
-
-    Strings extraArgs;
-    enum { cmdUnknown, cmdInstantiate } command = cmdUnknown;
-
-    char * homeDir = getenv(nixHomeDirEnvVar.c_str());
-    if (homeDir) nixHomeDir = homeDir;
-
-    nixDescriptorDir = nixHomeDir + "/var/nix/descriptors";
-
-    for ( ; argCur != argEnd; argCur++) {
-        string arg(*argCur);
-        if (arg == "-h" || arg == "--help") {
-            printUsage();
-            return;
-        } else if (arg == "-v" || arg == "--verbose") {
-            verbose = true;
-        } else if (arg == "--instantiate" || arg == "-i") {
-            command = cmdInstantiate;
-        } else if (arg[0] == '-')
-            throw UsageError("invalid option `" + arg + "'");
-        else
-            extraArgs.push_back(arg);
-    }
-
-    switch (command) {
-
-        case cmdInstantiate:
-            instantiateDescriptors(extraArgs);
-            break;
-
-        default:
-            throw UsageError("no operation specified");
-    }
-}
-
-
-int main(int argc, char * * argv)
-{
-    ATerm bottomOfStack;
-    ATinit(argc, argv, &bottomOfStack);
-
-    /* Put the arguments in a vector. */
-    Strings args;
-    while (argc--) args.push_back(*argv++);
-    Strings::iterator argCur = args.begin(), argEnd = args.end();
-
-    argCur++;
-
-    try {
-        run(argCur, argEnd);
-    } catch (UsageError & e) {
-        cerr << "error: " << e.what() << endl
-             << "Try `fix -h' for more information.\n";
-        return 1;
-    } catch (exception & e) {
-        cerr << "error: " << e.what() << endl;
-        return 1;
-    }
-
-    return 0;
-}
diff --git a/src/globals.hh b/src/globals.hh
index 8597ae2f89..8c6a763c48 100644
--- a/src/globals.hh
+++ b/src/globals.hh
@@ -21,7 +21,13 @@ extern string dbRefs;
    with hash h2.
 
    Note that a term $y$ is successor of $x$ iff there exists a
-   sequence of rewrite steps that rewrites $x$ into $y$. */
+   sequence of rewrite steps that rewrites $x$ into $y$.
+
+   Also note that instead of a successor, $y$ can be any term
+   equivalent to $x$, that is, reducing to the same result, as long as
+   $x$ is equal to or a successor of $y$.  (This is useful, e.g., for
+   shared derivate caching over the network).
+*/
 extern string dbSuccessors;
 
 /* dbNetSources :: Hash -> URL