In R7RS, page 14, section 4.2.1 Conditionals. The form 'cond-expand' can
accept following conditions

```
A <feature requirement> takes one of the following forms:

- <feature identifier>
- (library <library name>)
- (and <feature requirement> ...)
- (or <feature requirement> ...)
- (not <feature requirement>)
```

The form (library <library name>) is used to test whether a R7RS library
named <library name> exists or not. For example:

```
(cond-expand
 ((library (srfi srfi-1))
  (display "Yes, we have SRFI-1.\n")))
```

However, this form is currently not supported by Guile. Supporting this
form can help Guile use portable R7RS library more smoothly.

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