code: public void execute() throws MojoExecutionException {
          
          if (skip) {
              getLog().info("Skipping ceylon:sdk-check");
              return;
          }
          
          if (System.getProperty(CEYLON_REPO) != null) {
              getLog().debug("System property ceylon.repo found: " + System.getProperty(CEYLON_REPO));  
              sysRepo = new File(System.getProperty(CEYLON_REPO));
          } else {
              getLog().debug("System property ceylon.repo not found");
          }
          
          if (sysRepo == null && System.getenv("CEYLON_HOME") != null) {
              getLog().debug("Environment variable CEYLON_HOME found : " + System.getenv("CEYLON_HOME"));            
              sysRepo = new File(System.getenv("CEYLON_HOME"), "repo");
          } else {
              getLog().debug("Environment variable CEYLON_HOME not found");
          }
          
          if (sysRepo == null) {
              String dotCeylon = System.getProperty("user.home") 
                     + File.separator + ".ceylon";
              
              File hDir = findSdkHome(dotCeylon);
              
              if (hDir != null) {
                  File r = new File(hDir, "repo");
                  if (r != null && r.isDirectory()) {
                      sysRepo = r;
                  }
              }
          }
          
          if (sysRepo == null) {
              throw new MojoExecutionException("Ceylon system repo (ceylon.repo or CEYLON_HOME) not set,"
                  + " or repo not found in them, or in {user.home}/.ceylon/ceylon-1.x.x/repo. "
                  + "Please use goal ceylon:sdk-download");
          } else if (!sysRepo.isDirectory()) {
              throw new MojoExecutionException("Ceylon system repo found but does not exist or is not a directory");
          } else {
              System.setProperty(CEYLON_REPO, sysRepo.getAbsolutePath());
              getLog().info("Ceylon system repo set to " + System.getProperty(CEYLON_REPO));
          }  
      }
docs: Execute Mojo.
  @see org.apache.maven.plugin.Mojo#execute()
  @throws MojoExecutionException In case SDK is not found, or other error