3.4.7 `exist?' - test for value name

Usage: (exist? ag-name)
return SCM_BOOL_T iff a specified name has an AutoGen value. The name may include indexes and/or member names. All but the last member name must be an aggregate definition. For example:

(exist? "foo[3].bar.baz")

will yield true if all of the following is true:
There is a member value of either group or string type named baz for some group value bar that is a member of the foo group with index 3. There may be multiple entries of bar within foo, only one needs to contain a value for baz.

Arguments:
ag-name - name of AutoGen value