skip () {
    if test "$NIX_DEBUG" = "1"; then
        echo "skipping impure path $1" >&2
    fi
}


# Checks whether a path is impure.  E.g., `/lib/foo.so' is impure, but
# `/nix/store/.../lib/foo.so' isn't.
badPath() {
    local p=$1
    
    # Relative paths are okay (since they're presumably relative to
    # the temporary build directory).
    if test "${p:0:1}" != "/"; then return 1; fi
    
    # Otherwise, the path should refer to the store or some temporary
    # directory (including the build directory).
    test \
        "${p:0:${#NIX_STORE}}" != "$NIX_STORE" -a \
        "${p:0:4}" != "/tmp" -a \
        "${p:0:${#NIX_BUILD_TOP}}" != "$NIX_BUILD_TOP"
}