#;2> (define-class <a> ())
#;3> 
#;3> (define-method (foo (a <a>))
--->   (print "foo v1 on " a))

Note: implicitly defining generic-procedure: foo
#t
#;4> (define-method (foo (a <a>))
--->   (print "foo v2 on " a))
#t
#;5> 
#;5> (define a (make <a>))
#;6> (foo a)
foo v2 on #<coops instance of `<a>'>
#;7>