docs: Expression XreadSimpleValueSpecificationOrNull() {
  
          Expression e;
  
          switch (token.tokenType) {
  
              case Tokens.X_VALUE :
                  e = new ExpressionValue(token.tokenValue, token.dataType);
  
                  read();
  
                  return e;
  
              case Tokens.QUESTION :
                  e = new ExpressionColumn(OpTypes.DYNAMIC_PARAM);
  
                  compileContext.parameters.add(e);
                  read();
  
                  return e;
  
              default :
                  return null;
          }
      }