`lhs:rhs`

. These expressions are used to define 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 initial value. The
invocation can be accessed using the `DEF_INV_idx`

. The initial value
expression is accessed through `DEF_EXP_idx`

.