[PD] default abstraction argument(s)

[Federico]

my abstraction accepts two creation parameters ($1, $2).
i want to make $2 optional, but it should default to value 1 if $2 was
omitted.

i tried in this way:

 [loadbang]
   |
 [trigger bang bang]
   |     |
 [$2 ]  [1 ]
   |   /
   |  /
 [float ]

but if i omit $2, the float's value is 0 and not 1.
how to test if a creation parameter was specified?
pd doesn't provide a kinda of null-value?

-- 
Federico