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### Language

- `(compile-allow-cond-fallthrough [on?])` gets or sets a Boolean value indicating how to compile a `cond` or `case` expression without an `else` clause. If the value of this parameter is a true value, then `cond` or `case` expressions are compiled so that void is returned if no clause matches. Otherwise, `cond` or `case` expressions are compiled to raise the `exn:else` exception when no clause matches. Note that this parameter is used when an expression is compiled, not when it is evaluated.
- `(compile-allow-set!-undefined [on?])` gets or sets a Boolean value indicating how to compile a `set!` expression that mutates a global variable. If the value of this parameter is a true value, `set!` expressions for global variables are compiled so that the global variable is set even if it was not previously defined. Otherwise, `set!` expressions for global variables are compiled to raise the `exn:variable` exception if the global variable is not defined at the time the `set!` is performed. Note that this parameter is used when an expression is compiled, not when it is evaluated.