I thought (naively) that writing

\tl_set:Nx \l_tmpa_tl { \tl_to_uppercase:n {text} }

would put TEXT in the variable \l_tmpa_tl (as revealed by \tl_show:N 
\l_tmpa_tl). It doesn't, and nothing else that I've tried does either. 
What is the recipe for getting \l_tmpa_tl to contain TEXT, starting from 
text?

Andrew