On this page:

8 Unsafe for Clause Transforms

The syntax/unsafe/for-transform module provides a helper function that gives access to the sequence transformers defined by define-sequence-syntax. This is what the for forms use and enables faster sequence traversal than what the sequence interface provides.
The output may use unsafe operations.


(expand-for-clause orig-stx clause)  syntax?

  orig-stx : syntax?
  clause : syntax?
Expands a for clause of the form [(x ...) seq-expr], where x are identifiers, to:

(([(outer-id ...) outer-expr] ...)
 ([loop-id loop-expr] ...)
 ([(inner-id ...) inner-expr] ...)
 (loop-arg ...))

which can then be spliced into the appropriate iterations. See :do-in for more information.

The result may use unsafe operations.

The first argument orig-stx is used only for reporting syntax errors.