# [Agda] Understanding the pattern matching in proofs

Amila Jayasekara thejaka.amila at gmail.com
Fri Apr 26 03:22:14 CEST 2013

```Hi All,

I am still learning Agda concepts.

I am following [1] tutorial and tried out some of the exercises in it.

It defines sublist as follows;

infix 20 _⊂_
data _⊂_ {A : Set} : List A -> List A -> Set where
stop : [] ⊂  []
drop : forall {xs y ys} -> xs ⊂  ys -> xs ⊂  y :: ys
keep : forall {x xs ys} -> xs ⊂  ys -> x :: xs  ⊂ x :: ys

I need to prove \sub is transitive. So I understand that in the transitive
function I have to pattern match on stop, drop and keep cases. But I am
having difficulty relating pattern matching with my proof.

⊂-trans : {A : Set}{xs ys zs : List A} → xs ⊂ ys → ys ⊂ zs → xs ⊂ zs

⊂-trans stop stop = stop

⊂-trans p q with (drop p) (keep q)

... | xs ⊂ (y :: ys) | y :: ys ⊂ y :: zs = .xs ⊂ .zs
....
...

Appreciate if someone point out where I am going wrong and help me to
understand the thinking pattern.

Thank you
Regards,
Thejaka Amila

[1] http://www.cse.chalmers.se/~ulfn/papers/afp08/tutorial.pdf
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