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The current proposal specifies that "error" is a procedure. It could instead specify that it is a special form. The main advantage of a special form is that it is possible to report the precise location information (file, line, etc) in addition to the error message. If "error" is a procedure, it is much harder to do so, in particular when "error" is tail-called as in: (define (f x) (if (< x 0) (error "x is negative") (+ 1 (sqrt x)))) The continuation that "error" will receive does not point back to "f", so I don't see how "error" could easily report that the error occurred in "f". Marc