In the diagram above, an arrow represents a hash function (from input to output). Actually, this particular function (Poseidon function) takes in two inputs to produce one output; where it appears that there is only one input, the second input is the appropriate hash of 0,0; let P(x,y) be the Poseidon function with inputs x and y, then a node that is shown to only have one child c at level n is actually the hash of c and the hash of the hash of … the hash of (0,0), such that there are n recursive levels of hashing (or n+1 total hash operations). This can be represented as P(P(P(P(...P(0, 0)...), 0), 0), c) or P(P^n(0,0), c) where the “exponent” represents recursion.