[PD] default value for an abstraction's creation argument

[robbert van hulzen]

dear all,
i'm wondering how to get a default value for an abstraction argument that is
not specified. eg: my abstraction [equality] can take 3 arguments, but i
don't want to *have* to type the arguments, in which case the abs should use
a default value.
i came up with the following solution:

[loadbang]
|
[$1]
|
[moses 1]
|      |
[t b]  /
|    /
|  /
|/
[f 440]    <-- default value

i'm not sure how elegant this is, really.
any thoughts?
cheers, robbert