This type of tree is used to represent expressions such as 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.