Merge commit '95aeb2ebae' as 'third_party/lisp/alexandria'

third_party/lisp/alexandria/features.lisp

@@ -0,0 +1,14 @@
+(in-package :alexandria)
+
+(defun featurep (feature-expression)
+  "Returns T if the argument matches the state of the *FEATURES*
+list and NIL if it does not. FEATURE-EXPRESSION can be any atom
+or list acceptable to the reader macros #+ and #-."
+  (etypecase feature-expression
+    (symbol (not (null (member feature-expression *features*))))
+    (cons (check-type (first feature-expression) symbol)
+          (eswitch ((first feature-expression) :test 'string=)
+            (:and (every #'featurep (rest feature-expression)))
+            (:or  (some #'featurep (rest feature-expression)))
+            (:not (assert (= 2 (length feature-expression)))
+                  (not (featurep (second feature-expression))))))))