* libguile/expand.c (compute_assigned, convert_assignment)
  (scm_convert_assignment): New functions.

* libguile/expand.h: Declare scm_convert_assignment.

* libguile/memoize.c (scm_memoize_expression): Do assignment conversion
  before memoization.

* test-suite/tests/syntax.test ("letrec"): Detection of unbound letrec
  variables now works.