Example: TWICE

Takes any number of forms and evaluates them all once and then all again.

> (define-macro (twice . forms)

(append '(begin) forms forms) )

> (twice (display "Macros are powerful")

(newline)

(display "Aren’t they?")

(newline) )

Macros are powerful

Aren’t they?

Macros are powerful

Aren’t they?

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