Boolean function
Leaves labeled with formula_1 (formula_2) represent the constant Boolean function which always evaluates to 1 (0). A leaf labeled with a Boolean variable formula_11 is interpreted as the assignment formula_12, i.e. it represents the Boolean function which evaluates to 1 if and only if formula_12. The Boolean function represented by a formula_3-node is the one that evaluates to 1, if and only if the Boolean function of all its children evaluate to 1. Similarly, a formula_4-node represents the Boolean function that evaluates to 1, if and only if the Boolean function of at least one child evaluates to 1. Finally, a formula_5-node represents the complementary Boolean function its child, i.e. the one that evaluates to 1, if and only if the Boolean function of its child evaluates to 0.