typedef enum {
    NullaryNode = 0,
    UnaryNode   = 1,
    BinaryNode  = 2,
    TernaryNode = 3
} NodeType;

typedef struct node_struct Node;
struct node_struct {
    double p;
    NodeType type;
    union {
        struct { // Binary/Unary
            Node *next;
            Node *prev;
        };
        struct { // Ternary
            Node *pathA;
            Node *pathB;
            Node *pathC;
        };
    };
};