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