Norman writes – “I added this web page just for fun. While looking at some proofs, it occurred to me that their structure resembled musical scores, so as an experiment I decided to see what they “sounded” like. Essentially, the musical notes correspond to the depth of the proof tree as the proof is constructed by the proof verifier. A fast higher note is produced for each step in the construction of a formula. A sustained lower note is produced when the formula is matched to a previous theorem or earlier proof step, to result in a new proof step (which corresponds to a proof step displayed on the Metamath Proof Explorer page that shows the theorem’s proof). Is it “music”? I guess that’s for you to decide. It is richly structured, with underlying themes that on the one hand seem to repeat but on the other hand are interestingly unpredictable, teasing your mind as the piece progresses.”Link.