} else if (!(USER_LEVEL.equals(sourceLevel) || GLOBAL_LEVEL.equals(sourceLevel))) {
                throw new IllegalArgumentException("sourceLevel must be one of: {" + USER_LEVEL + "," + GLOBAL_LEVEL + "}");
            } else {
                this.sourceLevel = sourceLevel;
                this.sourceLevelSet = true;
            }

    /**
     * @see java.lang.Object#equals(java.lang.Object)
     */
    public boolean equals(Object o) {
        if (this == o) {
            return true;
        } else if (!(o instanceof Extension)) {
            return false;
        } else {
            Extension e = (Extension) o;
            return java.util.Objects.equals(e.getArtifactId(), getArtifactId())

              <!-- was only a workaround for Plexus Container, hopefully never used by anyone else -->
              <exclude>org.apache.maven.plugin.DefaultBuildPluginManager#setMojoExecutionListeners(java.util.List)</exclude>
              <!-- could have never been used due to usage of non-export class (CoreExportsProvider) -->

        } else if (!(USER_LEVEL.equals(sourceLevel) || PROJECT_LEVEL.equals(sourceLevel)  || GLOBAL_LEVEL.equals(sourceLevel))) {
            throw new IllegalArgumentException("sourceLevel must be one of: {" + USER_LEVEL + "," + PROJECT_LEVEL + "," + GLOBAL_LEVEL + "}");
        } else {
            this.sourceLevel = sourceLevel;
            this.sourceLevelSet = true;