docs: private void lhs( RuleDescrBuilder rule ) throws RecognitionException {
          match( input,
                 DRL5Lexer.ID,
                 DroolsSoftKeywords.WHEN,
                 null,
                 DroolsEditorType.KEYWORD );
          if ( state.failed ) return;
  
          if ( input.LA( 1 ) == DRL5Lexer.COLON ) {
              match( input,
                     DRL5Lexer.COLON,
                     null,
                     null,
                     DroolsEditorType.SYMBOL );
              if ( state.failed ) return;
          }
  
          lhsExpression( rule != null ? rule.lhs() : null );
  
      }