FEATURES

Some feature/implementation differences between s1 and s2:
  Superset of s1:
  - s2 has gotos. (Won't be implemented in s1.)
  - s2 has o8, o16, o32, diff.
  - s2 has incr/decr and "+="-type operators.
  - s2 allows negation for overflow types.
  - s2 allows deftype names to shadow def names.
  - s2 doesn't sanity-check template defs' types.  (It should.)
  - s2 supports 16-bit literals and defs.

  Superset of s1, necessary to compile s3:
  - s2 has quick-return expressions (error handling with unary "#")

  Subset of s1:
  - s2 disallows assignment except for top-level exprs.
  - s2 disallows strinits nakedly in conditionals/swartch locations.
  - s2 enum constructor type-checking/lookup is different.
  - s2 disallows whole field lookup.
  - s2 requires expression statements be void.

Some feature/implementation differences between s2 and s3:
  Superset of s2, necessary to compile s4:
  - s3 has generalized array indexing.

Some feature/implementation differences between s3 and s4:
  Superset of s3, necessary to compile s5:
  - s4 has foreach loops.

Feature/implementation differences (to speak of) between s4 and s5:
  - (None.)