code: public void execute( EnforcerRuleHelper helper )
          throws EnforcerRuleException
      {
          this.log = helper.getLog();
  
          // if the file is already defined, use that. Otherwise get the main artifact.
          if ( files.length == 0 )
          {
              try
              {
                  MavenProject project = (MavenProject) helper.evaluate( "${project}" );
                  files = new File[1];
                  files[0] = project.getArtifact().getFile();
  
                  super.execute( helper );
              }
              catch ( ExpressionEvaluationException e )
              {
                  throw new EnforcerRuleException( "Unable to retrieve the project.", e );
              }
          }
          else
          {
              super.execute( helper );
          }
  
      }
docs: /*
  (non-Javadoc)
  
  @see org.apache.maven.enforcer.rule.api.EnforcerRule#execute(org.apache.maven.enforcer.rule.api.EnforcerRuleHelper)