defining `app' breaks modules

[Kevin Ryde]

In guile 1.6.3 on a recent i386 debian, I was foolish enough to make a
define of "app", promptly breaking use-modules.  For example,

        (define app "my application name")
        (use-modules (ice-9 rdelim))

gives

        standard input:2:1: In expression (eval-case (# #) (else #)):
        standard input:2:1: not-a-record "my application name"
        ABORT: (misc-error)

It'd be nice if the module system implementation could protect itself
against such stray defines.  I guess there's a lot of things in
boot-9.scm that could be upset, but "app" seems a particularly
unfortunate one, since it's short and rather natural for some sort of
application thing.