As I was learning LaTeX3, I stumbled upon some information missing
from the docs. Apparently the \cs_new: functions (and friends) do not
handle expansion for you just because you use 'x' in your argument
specifier. They should be used only to define the base function and
variants should be defined with \cs_generate_variant:Nn.
I feel like this should be part of the docs, since it's a pretty basic
rule. I've attached a small patch. Tell me what you think.