Scieneer Common Lisp 1.3.9 online documentation

set-macro-character char function &optional non-terminatingp readtable[Function]

Causes char to be a macro character which invokes function when seen by the reader. The non-terminatingp flag can be used to make the macro character non-terminating. The optional readtable argument defaults to the current readtable. set-macro-character returns t.