  We study embeddings of programming languages into one another that preserve
what reductions take place at compile-time, i.e., staging. A certain condition
-- what we call a `Turing complete kernel' -- is sufficient for a language to
be stage-universal in the sense that any language may be embedded in it while
preserving staging. A similar line of reasoning yields the notion of
safety-preserving embeddings, and a useful characterization of
safety-universality. Languages universal with respect to staging and safety are
good candidates for realizing domain-specific embedded languages (DSELs) and
`active libraries' that provide domain-specific optimizations and safety
checks.
