syntax-rules and conditional define

[Kevin Ryde]

In guile 1.6.3 on a recent i386 debian, I thought to make myself a
"define if not already defined" using

    (use-modules (ice-9 syncase))
    (define-syntax define-maybe
      (syntax-rules ()
        ((define-maybe name value)
         (if (not (defined? 'name))
             (define name value)))))

    (define-maybe x 123)

but run with "guile -s foo.scm" gives

    /down/guile/usr/share/guile/1.6/ice-9/syncase.scm:130:16: In procedure 
scm-error in expression (scm-error (quote misc-error) who ...):
    /down/guile/usr/share/guile/1.6/ice-9/syncase.scm:130:16: invalid context 
for definition of x

I guess I'm not sure if this is a bug, but

    (if (not (defined? 'x))
        (define x 123))

worked at the top-level, so I thought perhaps it could be expected to
in a macro too.