clojure lib

A lib is a source file located at a path that corresponds to a symbolic name.
By convention, a lib creates a namespace of the same symbolic name.

The require function loads a lib by its symbolic name and ensures switch back to namespace before the load.

(in-ns 'foo)
(require 'clojure.java.io)
; loads clojure/java/io.clj
; ... foo is still current namespace