user=> (source clojure.core.protocols/coll-reduce) 源未找到
但是，由于协议函数的var的元数据指向协议var，并且协议var知道它被定义的文件和行，因此可以通过提高‘source’功能来使其看起来像这样
user=> (source clojure.core.protocols/coll-reduce) (defprotocol CollReduce "Protocol for collection types that can implement reduce faster than first/next recursion. Called by clojure.core/reduce. Baseline implementation defined in terms of Iterable." (coll-reduce [coll f] [coll f val]))
