6F. default arguments to functions

default arguments allows you to specify for one or more
arguments of a procedure which is the default value, if the
procedure is called with less args than parameters. for example,
a procedure like:

PROC bla(a,b=1,c=NIL)

can be called like:			is equivalent with:

bla(a,b,c)				bla(a,b,c)
bla(a,b)				bla(a,b,NIL)
bla(a)					bla(a,1,NIL)

This can be useful and also express something about the
procedures function, i.e. that most of the time one would
call it with NIL anyway, so why not leave it out for clarity.
That's also why you should not overdo it with D.A.: do not
start specifying non-sensical values for procedures
out of pure laziness, if you feel a certain parameter really
has no default value.

to make calls with fewer args unambiguous, D.A. declarations
can only apply to the last 0..n parameters of a PROC of n parameters.

for example, illegal is:  PROC bla(a,b=1,c)

(you should then simply reorder the parameters, of course).
arguments supplied in a call are filled in from left to right,
missing arguments are added with D.A.'s as needed.