C functions, variable number of params?

How do I define a function as taking a variable number of parameters. The
documentation seems to indicate (...) but, no such luck.

markw=# create function concat( ... )
markw-#         returns varchar
markw-#         as '/usr/local/lib/pgcontains.so', 'concat'
markw-#         language 'c' with (iscachable);
ERROR:  parser: parse error at or near "."