运行命令mvn -v，没关系。但是运行命令mvn system:help：
[INFO] BUILD FAILURE
[INFO] Final Memory: 11M/153M
......
[ERROR] No plugin found for prefix 'system' in the current project and in the plugin groups [org.apache.maven.plugins, org.codehaus.mojo] available from the repositories [local (C:\Users\Administrator\.m2\repository), nexus (http://localhost:8081/nexus/content/groups/public/)] -> [Help 1]
org.apache.maven.plugin.prefix.NoPluginFoundForPrefixException:
为什么有一个前缀＆＃39;系统＆＃39;？怎么办？