Today a very large and rapidly growing amount of chemical knowledge is available through online databases. Reaxys (1) alone contains over 40 million reactions and in excess of 105 million compounds . Yet, this is only a fraction of the possible chemical space: it is estimated that 10 60 drug-like molecules are synthetically accessible . Modern high-throughput screening can test "only" in the order of 10 6 molecules in the lab , leaving a large gap for which tools are required to guide experimentation . Without algorithmic use of chemical knowledge, capable of navigating within the whole of known chemical space, our efforts would continue to be limited as illustrated by the fact that in medicinal chemistry, as an example, few heavily used reactions have dominated the chemical landscape with no additions to this set in the past twenty years .