`lhs:=rhs`

. These expressions are used to assign something
in Pico. Hence, the tree needs 2 children: left-hand-side being defined (in Pico
this is called the invocation, and can be `v:=...`

, `t[23]:=...`

or `f(x,y):=...`

) and the right-hand-side being the value that is assigned.
The invocation can be accessed using the `SET_INV_idx`

. The expression
representing the value that is assigned is accessed through `SET_EXP_idx`

.