I have yet another idea.

(SYNTAX-SEQUENCE <clause> ... <final>)
  <clause> ->
      (COMPUTE <pattern> <syntactic computation>)
    | (LET <pattern> <form>)
    | (<syntactic computation>)
  <final> -> <syntactic computation>

COMPUTE binds a pattern, a SYNTAX-RULES pattern, to the result of a
syntactic computation.  LET simply binds a SYNTAX-RULES pattern with a
form, not a computation.  The third kind of clause ignores the result
of a syntactic computation.  The result of the SYNTAX-SEQUENCE form is
the result of the FINAL.

I also have a suggestion to rename SYNTAX-BIND to SYNTAX-EXTEND, as the
monadic >>= operator is often differently named.