XSLT转换：Concat，Substring和Lowercase。无法让所有三个人一起工作

I am trying to use xslt to transform xml so that the output can be a username. I want to put the letters "gg_" and then the first letter of the first name and then the entire last name. I want the entire string to be in lowercase. I have successfully been able to concatenate, and use substring, but when I add in the lowercase my code doesn't work. I am trying to use xslt to transform xml so tha