Trying to parse operand for ":" operator

[#1]

Hello, I have the next line of code: 

g_list[j] = dt << Group Columns(: || char(test_list[j]) || char(_) || char(s_list[1]), Length(s_list));

The line is generating the next problem: 

Unexpected "||".
Trying to parse operand for ":" operator

What is the best way to fix the issue?

Thanks a lot

[#2 · Accepted solution]

I found the problem:
There is no need to parse the  : operator. The way I should do it is like that: 

g_list[j] = dt << Group Columns(:(test_list[j] || "_" || s_list[1]), Length(s_list))