t types for specific programs at will. A custom
argument type is required if you need to handle a data type that is
not supported natively by the argtable library. It involves defining
a new <I>arg_xxx</I> structure and writing the callback and
constructor functions that go with it. If you merely wish to alter
the parsing and validation rules applied to an existing <I>arg_xxx</I>
data type then hooking your own custom callbacks into the existing
structure as described in the previous topic is more appropria