create type input and output function examples

I've seen the docs for create type and an example of the syntax to create a 
type. What I haven't seen is the functions that are passed for the input and 
output elements.

CREATE TYPE box (INTERNALLENGTH = 8,   INPUT = my_procedure_1, OUTPUT = my_procedure_2);

Now what would be in my_procedure1 and my_procedure2 procedures? I know they 
are just example placeholders above but I need to see a real example of what 
would be in those procedures to better understand what they should do. 
Thanks.

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