Due to the binary architecture and the commutative nature of addition and multiplication, the expression tree contains many mathematically invariant models (symmetries). The design of the formulation should therefore impede redundancies. Eqs. 18-19 resemble cuts in the tree such that, if a unary operator is selected, the unused part towards the right children node is set to zero . The Eqs. 20-23 assure that if an operator is selected on a lower layer of the expression tree, there is an operator attached to the parental node . Likewise, it ensures that the children of a node with value zero also have no operator or variables attached.