fmgr.h - it's discouraged to access fcinfo directly but there is no
macro to get the number of arguments passed to the function. Checking
the number of arguments is often useful when you have a function which
can be called like:

 func('arg');
 func(null);
 func();

all mapping to the same C function.

the macro has a function-like appearance to match the other PG_*
macros.

Lee Kindness.