Alternative Triggering of YAsnippet Expansions

[Nordlöw]

I prefer to have my dabbrev-expand functions placed before yas/try-
expand() in hippie-expand-try-functions-list. This makes hippie-
expand() steal the expansion from yas/try-expand() and instead expand
if/for/while/... as a symbol. This because my buffers often contain
many symbols beginning with the C keywords if, for, while, else, etc..

But for the other cases where I really want the expansion of C-
keywords if/for/while/... how do I make YAsnippet expansion trigger on
keyword + whitespace + '(' instead? In this way I would get best of
both worlds; both dabbrev expand and yas/expand() the way I would like
it to work.

Thanks in advance,
Nordlöw