Like command-line, but with a few important differences. In addition to the normal options for flag-clause that command-line allows, a flag clause may also be of the form (begin-flags flag-clause ...), which has exactly the same semantics as if each flag-clause contained within begin-flags was spliced into the surrounding list of flag clauses. Additionally, command-line-ext supports command line flag expanders, which are similar to match expanders and allow a user to extend the grammar of flag-clause. A custom flag-clause created with define-command-line-flag-expander can expand to multiple flag clauses by using begin-flags.

Binds id as a command line flag expander. When (id expr ...) appears as a flag-clause in command-line-ext, a syntax object representing (id expr ...) is passed to transformer-expr, which must be an expression that evaluates to a procedure of one argument at phase 1. Whatever syntax object transformer-expr returns is then used in place of (id expr ...) as a flag-spec.