[Click] Solution to "Trace Analysis" problem in tutorial

[Ludovico Cavedon]

Hi everyone,
I was reading Click Tutorial problem set 1, and I could not understand
one line in the solution to problem 9 (Trace Analysis).

In particular:
...
   -> ct :: Counter
   -> shunt :: input -> output
   -> sa :: AggregateIP(ip src/8)
...

What is the use of "input" and "output" outside an "elementclass" context?

Moreover, if I run the code I get:
<expr>:6: unknown element class 'input'
<expr>:7: undeclared element 'output' (first use this block)

Thank you,
Regards,
Ludovico Cavedon