Out of date GATlab examples

There are tutorials that have out of date code such as the following GATlab code in the tutorial on preorders

```julia
@theory Preorder{El,Leq} begin
El::TYPE
Leq(lhs::El, rhs::El)::TYPE
@op (≤) := Leq

# Preorder axioms are lifted to term constructors in the GAT.
reflexive(A::El)::(A≤A) # ∀ A there is a term reflexive(A) which implies A≤A
transitive(f::(A≤B), g::(B≤C))::(A≤C) ⊣ (A::El, B::El, C::El)

# Axioms of the GAT are equivalences on terms or simplification rules in the logic
f == g ⊣ (A::El, B::El, f::(A≤B), g::(A≤B))
# Read as (f⟹ A≤B ∧ g⟹ A≤B) ⟹ f ≡ g
end
```

which should be

```julia
@theory Preorder begin
El::TYPE
Leq(lhs::El, rhs::El)::TYPE
@op (≤) := Leq
reflexive(A::El)::(A≤A)
transitive(f::Leq(A,B), g::Leq(B,C))::Leq(A,C) ⊣ [A::El, B::El, C::El]
f == g ⊣ [A::El, B::El, f::Leq(A,B), g::Leq(A,B)]
end
```

This and related code blocks should be brought up to date