To test our hypothesis, we evaluate how the number of compatible partners for a building block varies with the P(active) value of the building block. To calculate the number of compatible partners, we first identify all compounds that are active when a building block is in a certain position. We then count how many unique building blocks are in the other two positions from this list of active compounds. For active molecules, the number of compatible building blocks in position p j , N ij , such that building block x is in position p i is